You really won't know how it will change *your* body until you stick with it for a while. Even so, here's what I remember most about how spinning 3-4 times a week changed my body a couple years ago:
1. Oh . . . my . . . goodness! I had no idea my body could sweat so much!
2. I lost more body fat and gained more lean muscle with Spinning than any other type of exercise I'd tried before.
3. It made my glutes felt strong and toned, but they didn't appear to get any bigger.
4. My quads, hamstrings and calves appeared to get *slightly* bigger, but only because they got more muscular in a pretty, powerful-woman kind of way. When those muscles got super shapely, they helped draw unwanted attention away from my bum. Ahhh, those were the days!
Taking Spin classes as often as 4 times a week never made me feel un-girlie or like a big-butted embarrassment at all. But I guess the trick is to start out gradually and notice how you feel about the way it makes *your* body change. If you like it, keep it up! If you don't like the results, trust me, they fade as soon as you decide not to go to class anymore.
Hope this was helpful. Good luck, and thank you
for inspiring me to get back into spinning!