I'm currently trying to gain some size. I've read and been told by numerous people, that I just need to take in a lot of good carbs, a lot of protein, and some good fats. I've really been doing well at this. Occasionally I have something sweet, but not very often.
I've gained around 7-10 lbs on my morning/evening weights. Which isn't the bad part.
The bad part, is that my stomach seems to be getting much bigger. I have started running to try to burn some of this off, but am not sure why its happening in the first place.
The sad truth is that to truly gain muscle mass, you have to gain some fat as well. People usually work in phases; weight gain, then cutting, then weight gain, then cutting. Professional bodybuilders are often only in truly cut shape during competitive phases, the rest of the time they have bodyfat because they are gaining.
It is possible to keep fat gain to a minimum but you won't be maximising your muscle gain if you do. That has to be your choice. If you find it possible to diet and cut, then you shouldn't worry about the fat and concentrate on the muscle gain, if you find dieting difficult then you should keep an eye on the fat gain and act accordingly.
Only you can know what your genetics, willpower and motivation are. Unfortunately this is a situation where you can't have your cake and eat it!!!