This is where I'm confused. I'm no fighter. I've never had a weight cut. But, I was under the impression, that not going through a harsh cut is better for your energy levels. Penn *should* have more stamina at WW, right? OR have I got it backwards? I'm open to some education.
It really varies a lot. There are tons of factors to take into account. Everyting from muscle mass to hydration, cardio to potential illness.
With muscle mass its either there's too much mass and the fighter's unable to properly perfuse the tissues and they encounter lactic acidosis. Or they have too little muscle mass and their muscles fatigue faster because they're not adaped to the inceased weight of the opponent.
With hydration there's the question of how much was cut for the weigh in, how much was reintroduced during the interveining evening before the fight. There's a point where no matter how hard to try, if you've cut enough you physically can't rehydrate sufficiently. And whats more is that the body sometimes doesn't notice if one is dehydrated when they're no longer in the danger zone for dehydration. The fighter might feel great only to reenter the danger zone and gas within 5 minutes, but that's more of a coaching failure rather than a fighter failure.
Cardio is something that's very hotly debated. Some people believe sprints and distance cardio is enough of the right kind of cardio, some people believe sparring/rolling develops enough. While a third group believe a happy medium between the two is where the best cardio comes from. I'm honestly not sure which works better because there really isn't really ONE that's better. Some forms work better for different fighters. That's on eo fthe reasons why you'll see a fighter in one camp fail in the cardio dept, leave that camp and all of a sudden have a massive gas tank.
Illness is another thing thats debated. How dangerously does any illness affect a fighter's cardio? Damn dangerously. A head cold is the difference between a 5 round battle and a TKO loss. Look at Roy Nelson he had walking pneumonia against Mir. He looked like a zombie after about 30 seconds. (Still did a crazy amount of damage for someone who got dominated though).
For Penn against Fitch, I suspect he had too little muscle, ineffective cardio management in camp, and may not have been properly rehydrated. But that's just me.