Why were prehistoric sea creatures so tiny?

My friend Toby Lowther wrote to me back in December to ask this question:

As far as I understand it, the general rule for extant species is that it’s much easier to get much bigger underwater than on land, due to the role that water plays in supporting large bodies. But as was recently pointed out to me by a friend, sauropods seem to be much, much larger than any known or recorded prehistoric sea creatures — and, in fact, despite sauropods being much larger than any modern megafauna, plesiosaurs, mosasaurs, etc. seem to be smaller than, say, blue whales. Is there a current theory/explanation as to why? Why you have these various periods where land megafauna ends up larger than any contemporaneous sea megafauna, and why despite dinosaurs getting so big, the largest prehistoric sea megafauna are smaller than the largest modern whales?

It’s strange, isn’t it? The last I knew, Shonisaurus was the largest ichthyosaur, at about 20 m and 50 tonnes, and this is considerably bigger than any plesiosaur or mosasaur I know of. It’s up the sperm-whale size category, but not even close to the bigger baleen whales. Why not?

Meanwhile, there is solid evidence for sauropods massing more than this on land — and less solid but I think still good evidence for them getting at least twice this heavy.

So what’s the deal with all those Mesozoic sea creatures being so tichy compared with terrestrial beasts that had so many more size-related problems to deal with?

Speak, O commenters! We’re listening.