Here is an example of a mod that does that. Like I said, mostly the available mods seem to be more focused on variation in gear and facial appearance. I personally have not seen one single mod that stretches the body weight variation to include the extremes on each end. There are mods that have attempted to edit large swaths of NPCs, for instance to improve their faces, increase the number of templates for more variation, or mix up their gear, but the more a mod tries to edit of the original game, the more possibility of mod conflicts, so it, for instance, would be hard to combine multiple mods editing NPCs to get the desired effect, for instance. Therefore, to give all the NPCs you encounter more varied body weight, you'd pretty much have to edit every single NPC in the game except for the few you leave with the current weight. The thing is that the NPCs in game are not randomly generated, as it may appear, but rather randomly selected from a collection of prebuilt NPCs contained the game's data.
