Hi! Yep, you can step too much, and get overuse injuries. I don't know about the bulking up of the thighs--I'm sure that would depend on your body type.
If you just love step and would miss doing it, you could try just removing the step and doing a tape on the floor. (I know, I know, I say this all the time and I'm sure people are tired of hearing it from me!) You can tape a rectangle down to help orientate yourself, although without it you can really expand your range fo motion. I know I have done my knees a world of good by only using my step once or twice a week. I'm still doing the tapes, just not using the actual step.
Have fun!
Wendy