I don't understand leg work outs!

BrunetteRunner87

So I kind of understand why people do adduction and abduction lifting (those machines where you spread your legs) but it seems like every other part of my legs are worked out when I do cardio (running, elliptical, etc). I mean without doing legs excersizes I still have a firmer butt and legs, wouldn't doing even more exercises just make my legs TOO muscular?