Can a kid be too weak?

Ugh late night over-thinking here.

I have two kids, both boys. My first will be 6 in September, my second just turned 3. Lately I’ve been concerned about my first’s strength. They’re both tiny and lean, always have been, but the last few months or so when I’m playing with them like on the floor kinda rough-housing I feel like my second is surpassing my first in strength. Not in size, my first is still a good 10 lbs bigger, but he can’t push and yank me around like my younger one can (which sounds strange but I hope being moms you get what I’m saying). And watching them swim now that it’s summer, my second is a stronger swimmer despite being smaller and younger. My first recently started gymnastics and he looks so behind his peers, not just the kids who’ve been doing the sport for years but also the other newbies, he physically can’t do things the other kids can (but he’s still having a blast trying). Idk. Is this just normal all-kids-are-different thing? Or something I should mention at his well visit? It seems weird my second is surpassing him already.