Oh come on, most fat people (unless you mean morbidly obese) can get around just fine. They can walk, run, jump, climb, etc. Just not as fast or as far. If you don't make the fat guy's power super speed and have him doing summersaults there shouldn't be any conflict.

And think about Professor Charles Xavier - he's in a wheel chair which is much more limiting than fat - unless its truely extreme obesity. I'm sure 100 pounds of fat wouldn't slow superman down at all - its just the mental image of a fat guy moving fast and with agility that seems less realistic than a guy flying around picking up trucks.