How do animals get to be just plain big?

A man-size sea scorpion claw? Yes, that extinct scorpion is a worthy companion to the frog from hell. As the BBC story tells it,

"The eurypterids are believed to be the extinct aquatic ancestors of modern land scorpions and possibly all arachnids (the class of animals that also includes spiders).

"The biggest scorpion today is nearly 30cm so that shows you how big this creature was," said Dr Simon Braddy from the University of Bristol, UK.

Why were many extinct animals so big? Opinions differ:

"The species existed during a period in Earth history when oxygen levels in the atmosphere were much higher than today.

And it was those elevated levels, some palaeo-scientists believe, that may have helped drive the super-sized bodies of many of the invertebrates that existed at that time - monster millipedes, huge cockroaches, and jumbo dragonflies.

But Dr Braddy thinks the large scales may have had a lot to do with the absence early on of vertebrate predators. As they came on the scene, these animals would have eaten all the biggest prey specimens. "

Maybe. But speaking of just plain big, today's blue whales are bigger than the biggest dinosaur ever was.

Then there is the recently discovered colossal squid:
New Zealand squid expert Steve O'Shea, from Auckland University of Technology, has described the squid as "a true monster." He told the BBC: "Giant squid is no longer the largest squid that's out there. We've got something that's even larger, and not just larger but an order of magnitude meaner."

So it seems that in the ocean today, life forms - whether they breath air or water, can still be just plain big.