You could eat more. I entered the marine corps at 140 lbs, 6ft tall. While in, I learned to eat and train correctly and 2 yrs later I was up to about 180lbs. A few yrs later, 240, with abs still showing... but that kind of size is hard to maintain without getting fat. I've settled in at around 205-210, and don't lift as heavy as I used to, but I still can. Google Mark Rippetoe, 5x5 if you're serious about putting on some size.