Why are there no lobsters the size of horses or horses the size of shrimp?
For the most part, you can split the creatures into two broad categories: "squishy outside" and "squishy inside". Bugs, lobsters, shrimp, and clams are all squishy inside: they use some sort of hard shell to protect their squishy bits, to hang their internal bits off, and I would assume, to act as leverage points for their motive parts.
"Squishy outside" creatures are like humans, whales, birds, and so on. They generally start with a rigid internal frame, and then hang organs from it, attach muscles to it, and so on.
Of course, there are plenty of non-plant living things that this categorization doesn't cover, or is vague about: what about water-filled rigid things like starfish, what about things that are just totally squishy like sea cucumbers, what about squids, cuttlefish, and so on that might have a rigid part, but use some sort of other structure (like tentacles) for things the bony animals use bones for? Worms? But for the most part, huge amounts of creatures fit clearly into one category or the other. It's still a useful distinction, I think.
But it seems like there are tons and tons of squishy inside creatures that are relatively small. Insects. Crustaceans. Whatever those little bug type things big whales eat are. Both in terms of total amounts of them on the planet and in terms of different types. The biggest one I can think of is a really big lobster. I've also seen some terrifying videos of huge CRABS picking their way around northern fjords, or shimmying down trees in the south pacific. The smallest are super-small. I don't know what they are but definitely sub-naked-eye visible. Aren't there some little mites that live on your eyelashes?
There are also a pretty decent amount of squishy outside creatures: lots of fish, all the birds and furry animals. The biggest ones in water are whales. The biggest on land are probably elephants, right? There are lots of examples bigger than all the biggest squishy-inside creatures I can think of. Easily. On the small side, the smallest I can think of is maybe some kind of mouse.
However, there aren't very many super-small squishy-outside creatures, and there are no super-big squishy-inside creatures. Why is this?
Besides being some sort of engineering or physical constraint, I considered the fact that maybe this was just a chance of evolution, but I looked up the page for "exoskeleton" and "endoskeleton" (there's barely anything in wikipedia for endoskeleton) and it says that at least "exoskeletons" evolved independently loads of times in totally different lineages. It doesn't say anything for endoskeletons. I'm not even sure if this is the right division of creatures. For instance, the exoskeleton page talks about lizards with armor plates, which in my classification, would probably still make more sense to look at as a squishy-outside animal. An armored lizard still has a spine, and so on. I also can't figure out where else to look this up.
In olden times, there were way bigger dinosaurs (squishy outside), but there were also way bigger dragonflies (squishy inside), right? So…what gives?
Why is this? Is there something in the engineering of squishy-inside/hard-outside creatures that doesn't scale up well? Why were there no herds of armor-plated ruminants grazing across the American plains in the 19th century? Why do you never find an infestation of thumb-sized gerbils in your attic?
It's hard not to notice that the big extant squishy-inside creatures either live in the water, or are probably super-closely-related to things that do. On the other hand, the biggest everything lives in the water, so maybe that's a red herring.
Is it that the squishy-outside/bones-inside system only evolved once, and that happens to be the system that most of the big creatures evolved from? (Except squids, and weird borderline cases like those enormous jellyfish colonies or corals or something) Or is it that over a certain size something stops working? And if so, what? Is this even the kind of question that is meaningful to biology (in that a model that produces testable predictions can be made)? Or is evolution just so complicated you have to throw up your hands and say "it's a large parameter space, we ended up on various local maxima and for the same reasons there's a dune here and not there."
If you know any resources where I can figure this stuff out for myself for the future (I have a lot of other questions about plants and animals and stuff) I'd appreciate that too.