That’s an interesting question. The constituents of their atmospheres are nearly identical too, so it is a good comparison.
In both cases, the lighter gasses like hydrogen are absent, but in Venus’ case, more of the heavier gasses remain. I think the key is that Venus is about 8 times heavier than Mars, so it would have 8 times the gravity, and thus retain more of those heavier gasses.
Half the distance, square the solar wind intensity.