The size on the label has become largely meaningless and it depends on where you live. They have vanity sizing and in some places as many people get bigger, the sizes have got bigger with the same label. Then in other places where the people are generally small, they have labels that go the other way, making them "feel larger". When I was a medium (a long time ago) I could only squeeze into an XL in Japan :D

When it comes to "size" of the individual though, it is about what feels good for the person. Gym can make people larger or smaller, and both can feel good.

It is great that you are thinking about these things now though, because the world is getting more "complicated" when it comes to kids maturing and it seems to be happening faster physically, but going backwards mentally. There is an age gap of sorts in kids - if that makes sense. Maybe I will write an article about it :)