John, the deep is the realm of the near inconceivably bizarre, with many animals thought to be living fossils or simple relicts of a time gone by (a snapshot into the past).
Gigantism is a thing of the past (for what reason I don't know); but historically large species ruled, even larger than Recent counterparts (so that physiological limitations probably do not dictate the upper size of Recent animals; something else is responsible).
I don't believe many massive giants remain undiscovered (as in 20-foot-long animals), but 'giant' is a relative term, so the discovery of 'giant' sea spiders, sponges, whelks, slugs, worms etc. is a possibility. Perhaps 'real giants' (of the 20-foot kind) survive in the deep, and have done for millions of years, as this environment has remained Earth's most stable in time and space (until recently, with the advent of deep-sea trawling). Therefore it might not be selective pressure that accounts for their size; rather temporal stability and the persistence of relictual 'giant' taxa. It's a stretch - I'm just playing Devil's advocate. (Less variation in habitat than their shallow-water or terrestrial counterparts, and accordingly less speciation (if habitat variation drives speciation).)
But here's another thought. There are many instances of speciation where taxa rapidly double or halve in size, rather than acquiring new characters or character states and gradually morphing in time and space. (Punctuated Equilibria, or whatever the term is they use today; like species the terminology also evolves.) Perhaps shallow-water environments do limit size (it would be hard to hide a giant squid in a rock pool, beneath a slab of rock or coral, or manoeuvre around the coast; Sperm Whales frequently strand close to shore ... a dead end in the old gene pool; the same applies to squid). Big things die when they hit something or strand; little things are less likely to fall into this trap; the gene pool perpetuates in the latter. Could the question really be why do we experience 'gigantism in open habitat', rather than 'gigantism in the abyss'? Really, 500 metres, the typical habitat depth of giant squid, isn't abyssal, and the juveniles still occur in the upper few metres, and subadults (1-2 metres) probably occur in the upper 30m. During the next interglacial, with elevating temperatures and melting of the ice caps, and the increase in ocean size and depth, might we experience even larger marine forms (and smaller terrestrial forms).
Perhaps it's a bit of all - temporal stability, relictual taxa and unobstructed habitat. Then again, perhaps there is some selective pressure, some physiological adapatation to being large (or attaining a large size not being a lethal character state for a deep-sea species - as in don't think of it as being an adaptation - think of it as being a non-lethal condition). Cooler temperatures are known to result in development of larger forms/individuals, taking more time to mature, and cooler temperatures are experienced at depth (of course we're verging on Lamarckism here - the acquisition of acquired characters/character states .... but I'm ok with that). There are a number of papers out there on the effect of temperature on size (particularly for the Antarctic).
A really good question; I haven't got a clue!