The mechanics of the pterosaur flight
How could they grow so huge?
Contrary to what you might think, the giant pterosaurs of the late Cretaceous could go ballistic in a second. This is because contrary to your another misconception, birds don’t take off as much using their wings as their legs.
A big bird needs strong legs to launch itself, but the lower part of the body becomes a dead weight when the bird flies. It’s this flaw in the overall bird design that keeps the birds small and not some physical law that postulates that big things can’t fly.
The pterosaur quadrupedal take-off relied on all four limbs in the manner of the vampire bat. Developing the upper part of the body and forelimbs was thus beneficial for both take-off and flight. There was no trade-off between launch and flight involved. And this is why pterosaurs could grow gigantic