I've always had lots of muscular bulk on my legs, even when I was overweight and had little upper body muscle.
My theory is that I grew up biking, and have always been a "sprinter" style cyclist, because I used the bike to commute to school, work, etc. It was all about sprinting to try to catch the stoplights while green, or sprinting and then cruising, because that's what seemed natural to me. Disciplined, steady high-cadence riding didn't just magically occur to me when I was 13.
I think that sprinting just taught my body, over the years, to keep the fast-twitch muscle on my legs, because maybe I'd need it again. Even at times when I wasn't biking at all, my calves and thighs are very large.
When I seriously train for cycling, it doesn't seem to increase leg mass at all, it just firms things up and bring out definition.
I can't say as I mind.
I guess the moral of the story is to get kids started biking early. And, as sicko says, maybe get them walking on their hands when they're off their bikes. They'll be set for life.