There could be two reasons:
1. Testosterone release
As far as I understand, when one works out, it provokes a testosterone release that promotes the repair of microtears in the muscles involved in the workout. But testosterone also promotes a systemic anabolism, making virtually all tissues grow, including the intestines. Hell, even some bones, like the jaw, grow under the influence of testosterone!
2. Increased food intake
One key factor for muscle grow is the availability of protein in the organism. Bodybuilders often eat a lot more than the general population so they are sure all energy requirements for the workout are met, and all protein and aminoacid requirements for the muscle repair are also met. I guess that eating a lot could have the same effect on the intestines as a workout on a skeletal muscle, making them grow to handle more work.
So when my weight increases, is that in part due to the size of my guts ?
Edit: I recently read about "GH gut", which would be a side-effect of GH abuse. This question is not about it, nor is it about any other substance abuse.