The characters of this story are "30-year-old elementary schoolers." That's the only explanation for the proportions and the clothing... Even the synopsis says Jubei's tits have gotten bigger but everyone is still in the same 6th grade. Go Nagai once again creates a plot that makes the least bit of sense but at least it's entertaining.
No, it says "kun". Kun and Chan aren't gendered. Chan is just a cute less formal form of san=Mr/Mrs/Miss. And kun is more formal when used by men for lower-status women and usually casual between men. There are many more nuances.