You're a girl, bulking up muscle is pretty hard to do... but with that theory, walking up the stairs versus taking the elevator should be bulking up people too, right? I lift pretty heavy and do not have bulky muscles. This is me being able to bench press 135lbs, do bicep curls at 25, shrugs at 50 (and I don't think I am bulky):
I know that doesn't show legs in baggy sweats, but they are not boyish looking either.. I don't lift light weights, do cardio at high resistance in spin, and basically abuse the daylights out of myself for fun
A lot of people use HIIT (High Intensity Interval Training) for maximum results, which is basically alternating spurts of high intensity along with lower intensity (so like, 4 minutes at almost your maximum with 3 minutes at a lower intensity) without letting their heartrate drop very low on the lower portions. In my spin, we would do (with 10 being your max) intervals for a lot of the time alternating between 8/9 and 4.