We humans are fascinated by large animals. We’re actually a pretty large animal ourselves. Even if you ignore microbes and only consider the animals we can see, most are small. There are over 350,000 described species of beetles, for example, and we tower over beetles.
“For well over a century, zoologists and explorers to the poles have observed that organisms there can reach remarkably large sizes.” For mammals, it’s been proposed (Bergmann, 1847; Watt et al., 2010) that larger individuals are favored in cold climates since their smaller surface-area-to-volume ratios more easily maintain high, stable body temperatures – but that doesn’t explain giant cold-blooded creatures.
Before trying to explain a phenomenon, it’s useful to first consider if it really exists. There haven’t been rigorous studies to tell us if gigantism in polar and abyssal (deep cold) waters is a general trend of if we humans simply notice large animals.
As you might expect, the situation is not clear.
While you may think of polar bears or the extinct Ice Age megafauna, a recent article in The Journal of Experimental Biology looked at “organisms from the marine environment because the terrestrial Antarctic fauna is comparatively sparse, whereas the polar oceans are home to a fauna that is rich and often strange.” Giant marine animals include mollusks, crustaceans, worms, sponges, and trilobites (trilobites are a favorite of mine – alas, extinct but well known from fossils).
Some creatures seem to trend in the opposite direction, to smaller sizes in cold water. Dissolved oxygen concentration, which is higher in colder waters, correlates with size at certain taxonomical levels. Aqueous levels of calcium carbonate seem to play a role – sometimes.
“Despite a broad search for explanatory mechanisms, no consensus has emerged… Multiple factors probably contribute to gigantism in taxon-dependent ways. This prognosis could be viewed as gloomy, but it likely reflects the messy, historical, contingent nature of biology better than any simpler alternative.” Angilletta et al., 2004
So the story remains as murky as the oceans’ abyssal depths. But I’m only human and I notice the big guys – the Antarctic colossal squid (for example) is awesome, however it got so big.