My boy is the same. 10 years old, 58 pounds as of this morning and 4'6". You can see his ribs and he has the muscle definition that grown men work out for. He eats a TON. But... He does multiple sports and I don't remember the last time he sat in front of the tv or video game without either being sick or falling asleep.
I even had to provide a food and activity log for his ped!
It is never an insult but you also can't be surprised when people are shocked. P.S. I have a p'nut too and I would still be shocked if you told me that. I have an 18mo old still wearing 6-9mo shorts and 9 mo shirts. The 9 mo old I watch is the same size as him.