I feel like this got off topic.
I've been thinking the same thing ever since I started visiting this site. There seems to be an obsession with unrealistically large breasts here at F95. Now, I would never kink shame. I think as long as whatever you're into isn't harmful to yourself or others, do what you love! But if I had to break down the psychology of why people are having an interest in super sized breasts... Hmm...
Perhaps it has something to do with a tendency to overindulge in things that we like? I love powdered sugar on my pancake. It's delicious. But I know that too much will over sweeten it, make me fat and it'll drown out the flavor of everything else. I think that self control to enjoy things in moderation is... perhaps rare? People want bigger, better and more of what they like. Without restrictions, people can go pretty wild.
I could be way off here because I haven't done any research on this topic. And even if this is accurate, I'm sure there is more to it.