The nutrition-to-weight growth is supposed to be linear, but the actual calculation is a bit funky since I used integers for a lot of the weight stuff.
Basically, for every 3 nutrition, 1 lb is distributed to the belly, 1 lb to the thighs, and 3/4 lb is distributed to the chest (rounded down after sum).
This results in stuff like:
10 nutrition -> 8 lbs
11 nutrition -> 8 lbs
12 nutrition -> 11 lbs
I actually hadn't noticed any of this until you pointed it out, so good catch btw