The Mezzanine

This. The mystical connection explanation never occurred to me because I just assumed their relative strength was entirely dependent on the joke the writers wanted to make in that episode and has absolutely no bearing on how well-matched they are in past or future episodes.

ETA: As if we needed any more proof that the writers just make this shit up as they go along, check out how Olaf, in "Triangle," was turned into a troll by Aud, an ordinary human at the time. But by "The Gift," he's apparently a troll god, which makes his hammer the appropriate weapon to fight a god. So... ordinary humans can now create gods just by doing a spell? Um. Yeah. Okay. Maybe Buffy should've asked Anya to turn her into a troll god in order to fight Glory.