when comparing asses you gotta compare it with the ratio of their body, Nagatoro is small but her ass is quite plump when taking into account the rest of her body. The prez may be bigger but her ass is quite flat when looking at her overall appearance. Big tits but no ass? Nah n
I'm guessing you're basing that on that one picture in one of the extra chapters where the two are standing next to each other and Nagatoro is leaning over backwards?
The way I see it, whether she has proportionally more butt-depth or not is completely irrelevant because she has less butt-width. Her hips are narrower, even proportionally. So overall she has less butt than the President both in overall mass AND proportional to body-size. It is just arranged differently.
And in practice, the butt is just an extension of the hips, and if the hips aren't there then it just looks weird.
But I suppose yeah, the President needs to be more rounded dorsally, while Nagatoro needs to be more rounded laterally.