Kind of hard to say. In theory a bigger head and/or neck ought to be better, but it's no garuntee. I mean look at someone like Bob Sapp, his neck is bigger than my car, but when he gets hit hard he curls up in the fetal position and starts crying. Patrick Cote on the other hand, looks like a perfectly ordinary guy, but you could hit him 14 times with a two by four while he is asleep and he would wake up and kick your ass. I think a lot of it is just genetic/willpower.
lol bob sapp is a HUGE guy i love watching his huge self fight xD
personally the whole knockout thing for me isn't just from one punch most of the time. if you have a weak neck and your head turns or goes back alot its an extra second that they have to throw a punch before you get to see it. Having a strong neck is key in more ways than just with striking. Alot of it is tolerance for pain and how quickly you recover. i dont really think it matters how big your head is haha that sounds kind of silly