Well I've only come across three mods that do a sort of weight gain (each relying on something different)
The first is Gopher's Pumping Iron: http://www.nexusmods.com/skyrim/mods/29476/?
Obviously this is more of a Working out and Weight Gained by Muscle mass, so, not really what we want here
The second is Pork Up: http://www.nexusmods.com/skyrim/mods/23448/?
From what I gather is after Reading a book found on a Table in the Riverwood Trader, you start gaining weight at 5 per level (according to the mod at level 21 you've maxed out at 100)
Haven't really tested this so not sure if this still works or is completely broken now (The Reading of the book resets your weight to 0 and still works but haven't tried to level with it enabled)
The Third is found here on LL as a RND add-on: http://www.loverslab.com/topic/17160-weight-change-realistic-needs-and-diseases/
I haven't even remotely tried this one (just did a search for it more or less) but it sounds that using Realistic Needs and Diseases and this add-on that it tracts how much you eat vs, combat, Sprinting, over encumberedness, and length of time going without eating to affect weight gain or loss. If this mod still works it might be more in line with what's being asked.
This is just what I've been able to find, some others might be more able to say how the mod works (if they use or have used it) or if there's other mods out there that will affect weight gain in the manner in which you want.