I don't necessarily think it's as much of a weight issue as it is a lack of wrestling issue. The whole Cesar Gracie camp confuses me. Nick Diaz is one of the best strikers in MMA, regardless of how awkward it is. However, his wrestling is non-existant. Jake Shields is probably the best grappler in MMA. Both his wrestling and JJ are top notch. but his striking is as bad as Diaz's lack of wrestling. It's painful to watch. I've been taking TKD and kickboxing for 5 years and I honestly think I'm a better striker than Jake. Hell, I could outstrike Jake while pregnant I bet.
Nate Diaz needs to learn how to wrestle. In every sense, otherwise he's going to have trouble no matter what weight class he is in.