Your son is the same size my 20 month old son is. We took him to an endocrinologist and had a bunch of tests done (but only b/c metabolic disorders are in the family). Everything was normal. Henri is above average in many areas, verbal and fine motor skills being a few. He eats well, sleeps well, has tons of energy, and slowly but surely outgrows his clothing. I don't worry about Henri because we know that there is nothing wrong with him, he's just a tiny kid.
Something else you might want to consider is your own size, dad's size, and duration of breastfeeding/if you're still breastfeeding. I am convinced that Henri has grown to his *true* genetic potential. He was exclusively breastfed for 7 months and did not eat solids with regularity until about 13 months. We continued to nurse until he reached 17 months and he weaned on his own accord because I'm pregnant and I don't have any milk. He's never had a drop of formula in his life and did not start consuming cow's milk until he weaned from me. There was nothing in his diet that could have caused him to grow "more" than he would under natural circumstances. His 2.5yo brother on the other hand was switched to formula at 6 months of age because I'd lost my supply in pregnancy-and he was so young we had no other choice. He also started solids earlier than his brother. Paddy was bigger than Henri when he was Henri's age. Maybe its just because he got the "bigger genes", or maybe its because he was given things that made him grow unnaturally.
I wouldn't worry too much. I think there is too much pressure to have "bigger, taller, stronger" kids. I think Darwin was nuts and I don't agree with the whole "survival of the fittest" attitude that pediatricians seem to take with young children (or survival of the biggest, rather).