I got looking @ my dad's bikes and since he only rides 1 of them he said take any parts you want off them. I took one of the seats and a nice water bottle. I decided to give his bikes a try. What I noticed was it took a lot more "strength" to pedal my bike then my dad's. Each time I pedaled my bike my legs were engaged more like I was @ the gym vs pedaling a bike. This is because each time i pedaled, my bike would sinks down maybe 1/2 inch. This is what people mean when they say there is a "power lost" from having a rear suspension. I've grown to like this, because at the end of a bike ride my legs feel like I've worked out at the gym.

The weight lost is purely due to cardio, that and your legs make up such a huge portion of your overall body that when you work them hard you burn even more calories.