>>103384601
Big tits are a lot more flexible and forgiving design wise. They can be perky or sagging and can still work well. As long as they're shapely and form fitting and not look like giant balloon bolt-ons, they'll work with literally any design you can think of. Same goes for regular sized tits.

Pure, pancake flat chest is a lot harder to pull off. You pretty much have to design around the flat chest aspect. Character design has to be close to 10/10 cause you don't have any tits to draw focus on, so other aspects of the body will have to be the center of attention which is harder.