My LO was also "small" at 1 year. She doubled her weight in 6 months from birth to 6 months, and then at a year, hadn't gained much from 6 months. She was gaining in height though, and is a very tall girl. Still, the doctor was concerned, but I just kept trying to give her healthy things, and good fats, etc, as others have mentioned. Now she is still small but at least over 20 pounds...but I bet you she will turn 2 and still be about 22 pounds by the looks of things. I was told by another doctor that its the overall health to be most concerned about, some babies are skinny, others are not. My LO is obviously healthy and active.
Anyways I don't have a ton of advice, just that my LO was gaining slowly at 12 months, and now, although still small, seems to be gaining at a pace, just not as fast as others. They now say she is fine and healthy. Just to show you that someone else has a small healthy baby. Do what you can and keep monitoring it.