Does anyone else have a small baby? my son is 19 months old, 15 pounds 28 inches long?

he was born with a couple Cardiac issues that were resolved right before his first birthday with a surgical procedure, he was diagnosed with broncho pulmonary dysplasia also, but from a recent lung doc visit they said hes out grown it, he was born at 36 weeks gestation 4.1 pounds and 14 1/2 inches long, everyone says hes doing ok, he just needs to fatten up abit, but im just wondering if anyone else has gone threw this...thanks :) oh and hes also crawling and pulling himself up on stuff now too, and there's been lots of tests run on him and they said that all the tests keep coming back fine, so im just abit stumped :)