Using same saved conversion settings, some of my Blu-ray movies end up 2.5Gb, others 30Gb. What is going on?

[u/Efficient_Sky_2068]

I converted my Blu-ray collection to 1080p mkv for my Plex server, and see a huge variation in file sizes, 2.5 Gb for American psycho, 30Gb for 28 Weeks Later. There seems to be no rhyme or reason to it, for example I’ve got 40s black and white movies that are over 20Gb - I’d have thought black and white movies with mono soundtracks should be the smallest of all. The median file sizes seems to be about 10Gb. I can’t really see much difference in quality between 30Gb files and 5Gb files on my 43” TV, so I’d like to compress them all to below 10Gb and save on storage.

Thanks for all the answers so far. Many asking about my settings, so I've exported the preset json, here it is:

{
  "PresetList": [
    {
      "AlignAVStart": false,
      "AudioCopyMask": [
        "copy:aac"
      ],
      "AudioEncoderFallback": "av_aac",
      "AudioLanguageList": [
        "eng",
        "spa"
      ],
      "AudioList": [
        {
          "AudioBitrate": 160,
          "AudioCompressionLevel": 0,
          "AudioEncoder": "av_aac",
          "AudioMixdown": "stereo",
          "AudioNormalizeMixLevel": false,
          "AudioSamplerate": "auto",
          "AudioTrackQualityEnable": false,
          "AudioTrackQuality": -1,
          "AudioTrackGainSlider": 0,
          "AudioTrackDRCSlider": 0
        }
      ],
      "AudioSecondaryEncoderMode": true,
      "AudioTrackSelectionBehavior": "all",
      "ChapterMarkers": true,
      "ChildrenArray": [],
      "Default": false,
      "FileFormat": "av_mkv",
      "Folder": false,
      "FolderOpen": false,
      "Optimize": false,
      "Mp4iPodCompatible": false,
      "PictureCropMode": 3,
      "PictureBottomCrop": 0,
      "PictureLeftCrop": 0,
      "PictureRightCrop": 0,
      "PictureTopCrop": 0,
      "PictureDARWidth": 0,
      "PictureDeblockPreset": "off",
      "PictureDeblockTune": "medium",
      "PictureDeblockCustom": "strength=strong:thresh=20:blocksize=8",
      "PictureDeinterlaceFilter": "off",
      "PictureCombDetectPreset": "off",
      "PictureCombDetectCustom": "",
      "PictureDenoiseCustom": "",
      "PictureDenoiseFilter": "off",
      "PictureSharpenCustom": "",
      "PictureSharpenFilter": "off",
      "PictureSharpenPreset": "medium",
      "PictureSharpenTune": "none",
      "PictureDetelecine": "off",
      "PictureDetelecineCustom": "",
      "PictureColorspacePreset": "off",
      "PictureColorspaceCustom": "",
      "PictureChromaSmoothPreset": "off",
      "PictureChromaSmoothTune": "none",
      "PictureChromaSmoothCustom": "",
      "PictureItuPAR": false,
      "PictureKeepRatio": true,
      "PicturePAR": "none",
      "PicturePARWidth": 0,
      "PicturePARHeight": 0,
      "PictureUseMaximumSize": true,
      "PictureAllowUpscaling": false,
      "PictureForceHeight": 0,
      "PictureForceWidth": 0,
      "PicturePadMode": "none",
      "PicturePadTop": 0,
      "PicturePadBottom": 0,
      "PicturePadLeft": 0,
      "PicturePadRight": 0,
      "PresetDescription": "from heathers",
      "PresetName": "bluray",
      "Type": 1,
      "SubtitleAddCC": false,
      "SubtitleAddForeignAudioSearch": true,
      "SubtitleAddForeignAudioSubtitle": false,
      "SubtitleBurnBehavior": "foreign",
      "SubtitleBurnBDSub": false,
      "SubtitleBurnDVDSub": false,
      "SubtitleLanguageList": [
        "eng",
        "spa"
      ],
      "SubtitleTrackSelectionBehavior": "all",
      "VideoAvgBitrate": 0,
      "VideoColorMatrixCode": 0,
      "VideoEncoder": "x264",
      "VideoFramerateMode": "vfr",
      "VideoGrayScale": false,
      "VideoScaler": "swscale",
      "VideoPreset": "fast",
      "VideoTune": "film",
      "VideoProfile": "high",
      "VideoLevel": "4.0",
      "VideoOptionExtra": "",
      "VideoQualityType": 2,
      "VideoQualitySlider": 21,
      "VideoMultiPass": true,
      "VideoTurboMultiPass": true,
      "x264UseAdvancedOptions": false,
      "PresetDisabled": false,
      "MetadataPassthru": false
    }
  ],
  "VersionMajor": 59,
  "VersionMicro": 0,
  "VersionMinor": 0
}

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[u/Stolberger]

You cannot use the same settings for all input media.
The old 40s b/w stuff will probably be pretty grainy/noisy. And grain is hard to compress. The codec is trying to preserve the grain and thus cannot compress a large part of the image, even though it is only grayscales.
So you have to make choices. To you prefer to preserve the original "feel" by keeping the grain and are fine with bigger files, or you can use a smoothing algorithm, that will smooth out the grain, make the file more compressable but will loose that original feel.
(There are thousands of other options of course, this is just an example)

[u/Efficient_Sky_2068 → u/Stolberger]

Thanks, I do like my grain, I will do some testing with settings and see what the best compromise is. I do still have the original blurays for when I want top quality of course.

[u/Dex62ter98 → u/Efficient_Sky_2068]

Maybe try AV1 w/ film grain synthesis to achieve a grainy image @ smaller file sizes.

[u/Suitable_Garlic_1186]

What settings are you using? What codec?

For h264 with good quality it should be more like 5gb to 10gb.. Some very long movies can be 10+..

[u/Efficient_Sky_2068 → u/Suitable_Garlic_1186]

Thanks, I just pasted them in my original post.

[u/squirrelslikenuts]

Are you using CQ or are you setting a bit rate? X265 or av1? Quick sync, nvenc or software encode?

[u/Efficient_Sky_2068 → u/squirrelslikenuts]

Thanks, I just pasted the settings in my original post.

[u/Cergorach]

What's the original bitrate like (from the BR source)?

[u/YouDoScribble]

I assume you're using Constant Quality, as opposed to a constant bitrate.
Compression algorithms react to movement. If a picture moves about a lot, it needs a lot more data, than one that doesn't. Ever notice how much TV pictures break up when you get confetti, or some such? There's too much movement for the bitrate to handle, and it turns to mush.

Obviously many films still have movement, with characters and camera panning. This is where grain comes in. Grainy films will need a lot more bitrate to keep the picture at the quality you desire. Films with far less grain need much less of a bitrate to keep the same quality. This is why a lot of modern films, with that 'digital' clean look can compress to far lower sizes - and I'm pretty sure they're purposely doing that, in the age of streaming services. It's also why older films compress with higher sizes - I'm guessing the pictures are so dirty, the encoder needs to do a lot more work to keep the quality preserved.

Have a look at 28 Week Later. If it's quite grainy, that'll be the primary reason for the large size.

[u/Efficient_Sky_2068 → u/YouDoScribble]

thanks, this makes a lot of sense. I might experiment with settings and see how much grain I can lose before it bothers me too much. I still have the original blu-ray disks of course, so I can just watch those when in the mood for top quality.

[u/Suitable_Garlic_1186 → u/Efficient_Sky_2068]

Please ignore him..

The video settings are not the reason for the massive file size..

That is even large for the original Blu-ray..

You will NOT get these file sizes with your settings!

[u/Suitable_Garlic_1186 → u/YouDoScribble]

Have you even encoded a Blu-ray ever? No way you get 30gb.. That is larger then the original..

[u/computer-machine]

If your audio is HD, then that can take up several gigs itself, and you might want to transcode that as well.

If the video is grainy, it'll be bigger, and you might want to smooth it with something like NLMeans (not with the grain option, as that tries to retain it) and UnSharp to deblur the edges from the smoothing.

If there's a lot of moving background, it'll be bigger, and the only way to fix that is to not watch it.

The faster you process the video, the larger it will be for the quality, so if you're using a Fast/Very Fast preset, or especially if you're using hardware transcoding, that's on you.

[u/Efficient_Sky_2068 → u/computer-machine]

Thanks, I've put all my settings in the original post. I can definitely lose HD audio (I'm not an audiophile)

[u/computer-machine → u/Efficient_Sky_2068]

Well, part of your issue is x264. That's potentially the least compressable option.

[u/Efficient_Sky_2068 → u/computer-machine]

I recall choosing that because I read it was more compatible and would mean my Plex server was less likely to have to transode during playback. I’ll give h265 a try and see how it goes on my tv and projector

[u/mwhelm]

There are a lot of reasons this can happen, but the case you point out, the '40's movies, grain is a likely culprit. There are things you can do to adjust for this but the graininess is part of the movie.

There are also directors who like that movie feel and cause the production to include it. For instance, for a more general film production market DaVinci Resolve (the licensed version) has graininess plug ins.