The big guys being the earliest infected seems like a good idea. I was also thinking they had eaten other zombies and got more of a mutagen load that way. They seem the predators of the "new" food chain. That would explain why the regular zombies run from them unless their fearless leader is there.
One tentative theory I have is that if was a biological attack, one way to save on clean up is by having the zombies turn on each other when the food supply dries up. The zombies that survive are bigger and eventually can't sustain their bulk and starve (like the water zombies).
Another one is that it was an experimental gene therapy meant to cure mental illness that got out of hand. The smarter zombies were actually more mentally ill. I like that idea so much I'm almost hoping that's not the case.
I'm sort of expecting the
Skittles to have been one of the researchers or terrorists.