Spinning is a great exercise and, in fact, it is important to gradually increase the intensity just as you would with strength training to achieve the best training results. While scientific research does not support the notion that cycling activities make ones thighs and buttocks become disproportionately larger, we have heard of a substantial number of anecdotal reports, particularly from women, claim that biking increases the size of their hips, thighs, and buttocks - and we believe them. The best way to prevent such 'bulking up' is to vary your exercise routine. For instance, spinning classes may be your primary aerobic exercise but be sure to also include other types of aerobic exercise as well. A great way to do this is to include a session of walking, jogging, or using the stair stepper, elliptical trainer, or rowing machine at least once a week.