Yeah. I've been thinking about that. It seems that, in the AVN universe, girls grow their most interesting parts forever (and also their height) -- maybe that's the reason we don't see too many sexagenarians and above around... They're probably in their houses unable to move because their tits grew too big. And their faces must be devoid of any cuteness: they must have a stern and serious face, sometimes, with angular features.
It's even more noticeable in those "household" games wherein the younger "roomie" always looks small and cute while the older "roomie" is bigger and curvier (sometimes with Bella's proportions). This may be true when the younger one is like 12 or 13, but it makes no sense when they're all post-pubescent. A girl can be 16 (or even younger) and be bigger and curvier than a way older one.
But some players are to blame too. Look at the pathetic discussion I've seen around about "Josy looking too young and those who like them are 'paedos!'" Dude... I've had 30/40 years old girls who were more petite than Josy (and, even if I didn't, one just has to look around to see it's not true)... That makes no sense for anyone who's seen the real world.
Since those same players are the ones who like MILFs, and they also associate voluptuousness with MILFs (maybe because they never had -- or even seen -- them in real life...), the Devs are probably paying fanservice to them--and thus we have Bella, who could like the average college girl except for her extra-proportions.
P.S.: I'm not complaining about those games that ALL girls have disproportionate proportions because, then, it's an aesthetical decision of the Dev to make 'em ALL look like this. I respect whatever aesthetical decision a Dev makes -- when it's coherent with his designs --, even if I may not enjoy it; and I don't even care about the girls looking like this -- one of my favorite games has all the girls looking more caricatural than Bella. The question here is about the characterization of older women with voluptuousness. (And this part wasn't for you, Geralt; just a general disclaimer to avoid misunderstandings.)