I am a PT as well, and as LMIFunkie stated, BP is a Muscular endurance workout. The beauty of it is that is designed as interval training where you are working opposing muscle groups, challenging your cardiovascular system with the intensity of the leg and back tracks and building muscle that is needed for everyday activity. In terms of caloric expenditure, studies have shown the AVERAGE calories burned in one BP class is 473. This includes Exercise Post Oxygen Consumption (or the "afterburn").
No, you cannot change genetics but you can alter the cards your dealt with somewhat. You will see an improvement in your body composition (ratio of muscle to bone to fat). With Body Pump, it takes approximately six weeks for you to really start to notice a change. At that point, it is time to increase your loads and keep the body "guessing". This prevents the dreaded "plateau"!!
When I used to train on the weight room floor and did what I call "regular" lifting for my fitness competitions I got BIG and bulky!! I hated that look! Body Pump has allowed me to streamline my body and develop nice, toned muscles that don't look out of proportion to my frame. Add in your doses of cardio with sensible eating and BAM!!! You got it!
Don't be afraid of becoming the stereotypical "big and bulky" lifter. That won't happen in Pump and the muscle that you will gain will help you prevent osteoporosis, increase your metabolism, not to mention make you look good as well! You're on the right track! Keep Pumpin!