Two of my boys are built like little bears at just 5 years old

Two of my boys are built like little bears at just 5 years old. Big bones, big heads, broad shoulders and backs, thick legs, dense as bowling balls. Both my wife and I lean toward heavy musculature and natural athleticism, and they’ve inherited that from both sides. Every now and then I pick up someone else’s child, especially if it’s a little girl, and it feels like lifting a doll. No weight. No density. Even at this young age, the difference between an athletic boy and an athletic girl is stark.