In order to fulfill some of our patron's unit requests, I've had to begin statting out the sizes, carrying capacity, and other characteristics of very large creatures - rocs, dinosaurs, etc.
Since in my game designs I always try to be as grounded as possible, the formula I use to assess the value of an animal includes a factor for carrying capacity and a factor for years to maturity, both of which are impacted by physiological factors such as muscle mass and body form. This was easy for horses and camels. I wanted to bring a similar level of empirical study to the more fantastical creatures.
I decided to review the scientific literature to see what could be learned. "How much can an eagle carry? That might influence how much a roc can carry." I was of course familiar with the square-cube law, which states that as size increases, mass increases at a faster rate, imposing a limit on maximum size. And I was familiar with the idea that large animals can't fly because their muscles can't generate enough lift. So I knew a roc couldn't be "real".
What I didn't realize is that scientists have no real idea how dinosaurs or prehistoric eagles could be "real" either!
Consider Angelos C. Economis article on "The largest land mammal", where he demonstrates that a land animal cannot be larger than 20,000 kg: http://www.sciencedirect.com/science/article/pii/0022519381903076
That's interesting, because we have fossil records of dinosaurs five times as large.
Ah, you say! It's because big dinosaurs are amphibious - they live in swamps where the water would help support their weight. The problem is that the big dinosaurs show all the hallmarks of being terrestrial, not amphibious, and their fossils have never been found in swamps. Robert Bakker convincingly showed that dinosaurs weren't amphibious at all. But to this day, it's not clear how they supported their weight, pumped blood, or lifted their heads. (As the literature puts it, "dinosaur biometrics are not well understood." When a scientist says "not well understood", he means "baffling anomalies".)
It gets wierder. The largest birds today is the wandering albatross. It's so large that it can hardly fly, and it's generally considered to be at the upper limit of powered flight by birds. For centuries, eagle breeders have been trying to breed larger and larger eagles. They've never gotten them bigger than 25-30lbs. So what does one make of Argentavis Magnificens, a 150lb bird with a 7m wingspan?
Or, even more interestingly, what do we make of Quetzalcoatlus, who some experts think might have reached 15m in wingspan and 450kg in weight?
The scientific literature is beside itself over Quetzalcoatlus. Some scientists argue that it must have only weighed 70kg in order to fly - which physiologists then claim is far too skinny, as the creature is the size of a 1,000kg giraffe.
Others claim the creature must have weighed 450kg, and that it couldn't fly - even though everything about its body build, frame, wing span, and other characteristics suggest it *could*. So they more-or-less split the difference, call it 250kg, and say it could fly long enough to glide.
I won't even begin to get into the fact that dinosaurs like T-Rex were bipedal, and show some evidence for adaptions that would have let them *jump*, or the fact that there may have been pterosaurs larger than Quetzalcoatlus, or dinosaurs bigger than Brachiosaurus.
All of which is a very long way of explaining why, ultimately, I think I'm ok with the fact that rocs can fly, and dragons can be huge and fast, even though I can't explain how. If you ask me, I'm going to say "the flight mechanics of rocs are not well understood" and "the biometrics of dragons are not well understood".