The cover page implied Momo was bustier than
Hajime, and the second panel corrects that.
Largely just an effect of everyone standing at different angles relative to the viewer methinks...
Also fun fact, breast size isn't static throughout a woman's life, even after puberty. The cast is largely in their young adulthood, and the younger of them (Momo, Aoba, Nene, etc.) aren't even 100% done growing. Beyond that, a woman's breasts can go up and down in size over large enough periods of time.
...not that I think that's what's happening here.