Does being fat make someone buoyant? Would a 300 lb man float easily while 150 lb guy would struggle? Why do some float, some sink?

It depends on if they are floating in water, mercury, air or earth.

In the first two the 300 pounder would actually fare better.

 The 300 pound man lifts weights and works out, the 150 pound man has a beer belly and doesnt work out. Fat floats, muscle sinks. . ;P This post was edited by carbonproduct at September 14, 2018 7:18 PM MDT

A high percentage of bodyfat makes you more buoyant. In most cases the 300 lbs. man would float better than the 150 lbs. man, but that could be reversed in some rare cases. There are a few bodybuilders who weigh about 300 lbs. with very low bodyfat, and they could float better than a small man who was very fat for 150 lbs.