Three things affect your buoyancy. The amount of fat tissue in the body (fat floats), the amount of lean muscle tissue (muscle sinks) and the amount of air you can hold in your lungs (the bigger the balloon, the more it floats). People with more fat in their body float easier (not just those love-handles...we have fat all through our tissues, so you don't have to LOOK fat), but others may have lots of fat, yet sink. Why? They also have lots of lean muscle and dense bone too. Think of a life preserver. If a 200 pound man uses a child's life preserver, he'd likely drown, right? That's because he's got too much weight for the life preserver to hold. Same idea with fat tissue...it might hold you up, but not if you have too much muscle and bone.
making a small donation to science.ca.