I was talking to a friend recently about ways to rev up her cardio routine as she was getting bored with her elliptical/treadmill rotation. I suggested she take a spinning class but she rapidly replied, "Oh no way, I don't want my legs to get all big and bulky."
I completely understand why anyone would feel that spinning will make their legs bulk up since you are primarily working your legs the whole time, but the truth is it won't. Spinning is all about increasing and decreasing resistance while increasing and decreasing speed — there is no component of it that is geared at bulking up your legs. In fact it's quite the contrary, since the quick rotations will leave your thighs and calves lean and sculpted. Don't believe me? Next time you pass an avid biker on the road check out her (or his) legs. I'd be willing to bet they're lean and toned.
If you want to try it out, be sure to look for a free spinning class near you.