An extremely hungry Galactus, that is.

It depends on how hungry you mean. "Extremely hungry" could mean that Galactus is near death, which means that a skyfather could likely take him out.

As powerful as he needs to be to provide some sort of challenge to the heroes involved without completely obliterating them.

Seriously, it's pretty much a pot device. Galactus being "hungry" normally just means that he's going to be single-mindedly going after a planet for lunch, but otherwise should still dwarf guys like the Silver Surfer or Thor in power.

When he's actually starving, he can still range in power from being above any of his heralds, to weak enough for the Avengers to defeat him. It all depends on the story the author is trying to tell.

@johnnyz256: I would say starving, I guess.

So how did Thor beat him if a starving Galactus dwarfs him in power?

Gallery

He destroys Solar systems while extremely weakened and in galactus the devourer where he was dying he was still strong enough to the point where guys like Silver Surfer and Thor were nothing to him.

At his lowest a Mr. Fantastic + Thing combo was enough to knock him the hell out.