two things... fat guys are usually alot stronger than skinny guys when they've both done hardly any training...fat people get strong all over as they carry alot of weight around..
ever wonder why guys who train are strongest when they are off season?
also, yes, he was defending himself, but defence doesnt cover beating someone when they are down..thats when defence turns into assault.
the legal definition and ruling is "defence is enough force to prevent an attack, that is reasonable and appropriate".
assault is when someone is under no threat, or no perceived threat, and you are aggressive towards them.
in other words, defence doesnt cover you kicking some guys ass cos he tried to kick yours...unless of course u KO'd him when you were both going at it.