A co-worker of mine is well over 200 lbs. I was 185-ish. We were the same height and width. What made the difference was his weight was all muscle from amateur wrestling. Apparently Fat cells are very light compared to Muscle cells.
Being fat is unhealthy, so I guess you want to be stronger. The answer is additional protein and resistance training, such as lifting weights. That will build muscle, not fat, and make you stronger and bigger in a healthy way. Don't go overboard with the protein; too much is not good for you, but more will help put on muscle. If you really just want to get fat, eat more sugary and fatty foods.