THE SKELETON APPEARS
We have an older thread that was (basically) about it here:
https://bbw-chan [DOT] nl/gen/res/10098.html
The Cliffnotes version is, you're right that it's a combination of all those factors but seems to be heavily lifestyle-related. It seems to be, that women who gain through excess carbs, still excersise, and 'graze' instead of stuff themselves is the best option to being giggly and not rubbery. I think you can see some of these differences in comparing ChubbyChaquita's gain to Mochii's- not to ever claim one is more georgeous than the other!- but Mochii's gain has made her very belly-heavy, and for obvious reason; she packs the damn thing tight, and her body tries to process it as fast as possible to clear the stomach. Chaquita however, gets high and just keeps eating all day. I think Roxxie does something similar, but I believe she also tempers herself to keep from strechmarks from forming.
I have no evidence to support these claims but just visual observation, but it seems like the best method is to have your gainer begin light excersise daily on top of current ADLs, then eat more than usual but focusing on consistency rather than stuffing. A calorie tracker is a good tool here. Keep *something* in the stomach all the time but don't stress it, and watch as the folds and rolls pile on!