This is sort of a weird comparison. If Jose Aldo came in and fought some dude that just sat on the couch and weighed 255lbs. Obviously Jose Aldo kicks his ass brutally. Situations will vary wildly though, and size does matter in a fight. A bigger fighter that has some skill will be a problem for a much smaller person.

If someone has actually trained technique to punch, they will punch with more power than someone who has not. If someone has trained combinations, they are ready to break simple defenses with their punches. if someone is in shape they will be able to outlast some fatass that gases. If someone knows BJJ, they will be able to easily maneuver into a choke on someone that has no idea it is coming. Assuming you are fighting some fatass untrained doofus, the answer here is easy.