Are you wanting to gain weight or build muscle? Two different things really. I'm the same size as I was when I graduated high school in 73. If you saw me wearing shorts, you would say 'Are those your legs or are you riding a chicken?', so I'm skinny. I tried different things to gain some weight, just to get a little heavier, but nothing worked. If your weight doesn't pose a health problem, you probably shouldn't worry about it.