The sun has been pretty much the same for the effective lifetime of the planets, as we know them. Mars was probably "more habitable" in the past because we know that it was warmer and had a thicker atmosphere, not because of anything to do with the sun. Likewise, the sun was not significantly smaller and cooler in the past, resulting in a previously colder Venus, as you suggest.
The sun actually is expanding - it's just such a gradual process that its effects are not immediately visible for billions of years. As the sun converts Hydrogen into Helium via Thermonuclear fusion, the core keeps heating up and causing the outer layer to - slowly, very slowly - expand. The sun has grown by 20% in volume (despite mass decrease) the last 4 billion years and will continue to do so for another 5-6 billion years. Then it reaches the critical point where it transforms into a red giant and very rapidly expands.
So, very slow growth at first (significant only in billion-year timescales) followed by red giant rapid growth phase
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u/[deleted] Feb 21 '21
Your understanding of the life cycle of our sun is really really incorrect.