Doctor concerned for toddler size?

Hi. I have a 2 year old boy. He is 39.5lb and 100.5cm tall. He’s big. And he’s always been big! He was born a week early and 9lb10oz.

He’s very proportionate in size. Slim with thick legs. Very active and very capable! However the doctor wants to refer us to a paediatrician/specialist to just “be safe”. What could possibly be wrong? I’m not concerned I’m just curious what could be “wrong” to cause a child to be large? lol.

We’ve already seen this paediatrician after his 1 year appointment for the same reason and she didn’t seem concerned at all.

And before you ask, no.. we (his parents) aren’t that tall. We’re both 5’8/5’9. However, all the men in my family are well over 6’.