I have friends who are figure competitors and I, myself, have been lifting weights for YEARS and I can assure you that gaining size is HARD for women. That is, unless they are predisposed for gaining muscle quickly because of high testosterone levels, or they are taking steroids, or they are eating massive amounts of calories every few hours in addition to spending a several hours a day training as bodybuilders do.
For women especially, gaining fat is easier than gaining muscle. But the more muscle you gain, the easier it is to shed excess bodyfat.
So ladies, do not fear lifting "heavy" weights - especially in a group exercise class. The weights used in a group exercise class are great for conditioning. Especially when combined with bodyweight exercises. But you will never see someone training for a figure or bodybuilding competition relying on group exercise classes.
If you feel like you are gaining size from resistance training, grab a tape measure and track your measurements every few weeks to see what is really happening. Measure your thighs, waist and arms in particular. Document your eating habits. Are you eating more than usual? And more importantly, what are you eating and when?
Often when you create a more sculpted or toned physique, your clothes will fit differently. This doesn't necessarily mean that you're getting bigger. Your shape is simply shifting a bit. If there is a question about getting bigger, grab a tape measure and see what exactly is going on. In my experience, the mirror can be deceiving.
To read more about the benefits of women training more intensely and with heavier resistance, visit this link (NYTimes.com)