We are wiser now. Relaxing the abdominal muscles and breathing into the the lower dantian is quite a change for some people, it may make some people feel fat, or even reveal a little extra flesh that was previously "sucked in." But needless to say at this point in history, cultivating qi will not give you a pot belly. Eating too many greasy donuts is and has always been the most likely cause of that.
But if you look at videos of old masters on, for instance, DPGDPG's Youtube page you'll see quite a few with hunch backs. Age itself causes bone degeneration, and no doubt some of these masters have suffered from starvation, spinal injuries or worse. Still I'm suspicious.
Could the hunch back be from bad training they participated in at some time in their lives? If that is the case, I would hope they let their students in on their errors so that these mistakes don't get passed on to future generations.
I've seen a lot of martial artists who take the weight of their arms in their upper spines. With higher level martial arts it is important that the practitioner takes none of his own weight in his or her joints. All the weight of the arms and head should pour down through the body so that there is no pressure on the joints or the bones.
Update: I wrote this blog about three months ago. I thought it was too mean so I hid it. But now that it resurfaced on it's own (I gave it a date way in the future) I think it's good food for thought--even if most masters don't actually have hunched backs.