Let's take a moment to actually appreciate the humor of this picture.
Anyways, what if Batman was obese? Gotham would be in one hell of a disaster with all those villains running wild. There is always something going on so imagine how slow Batman would be to respond to the bat signal. By the time he arrived at the crime, the villains would be long gone. There would be no athleticism when it came down to fighting man to man. No city needs an obese hero to be patrolling the streets. Batman needs to be in shape to stop those villains creating chaos in Gotham City. Batman needs to be... well... Batman!