Good question, and I'm not really sure what the answer is, but I'll take a crack at it.
From the size of beaks Steve O brought to TONMOCON it looked like the colossal squid beaks were considerably bigger (around 3 or 4 times larger) than the humboldt squids, and remember that weight scales as the cube of length, all other proportions being equal, so if the beak is 3 times as long, one would expect the weight to be about 27 times as much. But I think you're right that they're out of proportion a bit: they're more in proportion to the size of the prey than the size of the squid. The size of a colossal squid's prey (Patagonian toothfish) is not so much larger than the largest prey items of the humboldts. Also, seeing the preserved humboldt and giant squids side-by-side at Mote really illustrates how much more muscular and well-armed the humboldt is for its size... they're well suited to going after big prey, and they hunt in packs.
On the flip side of that, cephalopods pretty much have to take small bites: a cephalopod's esophagus passes through the middle of its donut-shaped brain, so if it takes too big a bite, it could literally cause brain damage, so there's a not much benefit in growing the beak any larger than the brain. They seem to be very good at strategically using their beaks, both to inject venom and find chinks in the armor of crustaceans, since ancient cephs pre-date fish so they probably primarily ate crustaceans and trilobites. More recent cephs seem to be able to quickly disable vertebrates, too, as in the "octopus vs. shark" video that's around a lot (it's probably on youtube) where it looks like the octo knows it can disable the shark by biting its spine, although it may just have broken the spine by arm strength. Squids seem to favor repeated rapid biting, but really, by the time a prey animal has been pulled from the tentacles to the arms, it's pretty much immobilized, so the squid doesn't have to kill it quickly with the beak. As far as I know, squids don't have toxins that disable vertebrates like fish, but most cephs that prey on crustaceans have cephalotoxin that disables them quickly, too... I assume squids catching fish pretty much can just hold them in their arms and take bites as they want, and there's not much a fish can do, although there might be other ways squids kill or disable their prey rapidly.
So I guess my bottom line guess at the answer is that unlike the jaws of a shark, the squid's beak isn't really its primary attack weapon: the tentacles and arms handle the capture and subdual of the prey, so the beak just has to be big enough to bite into the prey and get to the tasty parts.