Size matters, but there are other important variables as well: 1) Technique, 2) Cardio, 3) Amount of fighting experience, and 4) Rules of the game (i.e., are we talking about a judo match, or a sumo fight, or a Muay Thai contest, or etc.? Or are we in a mixed martial arts fight and all moves are allowed?). Without all these pieces of information provided, it is quite hard to say for sure who will win. We can safely assume that the obese fighter will have much worse cardio, but if he is up against a normal-sized adult who has no training/less training in fighting, he can conceivably win by lying on top of the normal-sized guy and smothering him, or by cornering him and delivering punches, etc. There is an MMA fighter named Mark Hunt who is undoubtedly obese, but he has great knockout power in his hands and accurate striking, and he has managed to do a lot of damage to opponents who are in better physical shape.