Not necessarily. Dancers, for example, can have large, well-defined legs, and many don't cross train with weights. Or, at least, I don't. I mainly do Pilates. And while I'd never say I had monster legs, I can't fit into skinny or even "slim" jeans.
I couldn't tell you the science of it all, but I do know what I've done. Hundreds of different movements from plié variations to arabesques to simple leg raises forward and back to sauté jumps to etc. Challenging myself not with added weight, but with new or different forms and difficult positions.
Oh, and yoga. Especially from
this guy (Mark Gonzales) or
this guy (DuncanYoga) or
this guy. Been doing yoga like that since I was 10, and they used to have a daily yoga program on the Oxygen channel. I figure those guys I linked be doing something right, because you'll feel it in your legs after they're finished with you.