By contrast the largest pterosaur was Quetzalcoatlus northropi with a wingspan estimated around 36 feet and a weight around 500 pounds. (Fun trivia, its outstretched middle finger, half that wing, is the longest vertebrate digit in the fossil record.)
Why did pterosaurs get so much bigger? Because of a simple quirk of vertebrate genetics!
It turns out that there are things which are evolutionarily easy to change (like your overall size, or when you hit puberty) and things that are essentially impossible. One of the things that is essentially impossible is the sequencing of when different parts of your body hit growth spurts. Hands+feet grow relatively early, but arms and legs only catch up near adulthood. It's cute when puppies and human babies have big hands and feet but stubby arms and legs. However it has consequences for flying vertebrates.
Birds have a wind that is all made out of their arms. Bats have a wing that is half arm, then half hand. Therefore both birds and bats are unable to fly until near adulthood because their arms have not yet grown yet, so their wings can't support their weight. This imposes a huge parenting burden as the young have to be taken care of. And the bigger the adult size, the bigger the parenting burden!
But pterosaurs had a wing constructed entirely out of their hands! Because hands grow early, models say that pterosaurs should have been able to fly from birth. Fossils of juvenile pterosaurs that died at sea confirm that they actually could. And because of that a large pterosaur would not impose a parenting burden, which means that pterosaurs were free to grow much larger than birds or bats.
This did not become important until birds evolved. Animals tend to compete through specialization. Each species focuses on what it is best at, and then they coexist. But feathers are more aerodynamic than a pterodactyl's fur. Therefore birds flew faster and farther while using less energy. The only thing that a pterosaur was better than a bird at was being big. So after birds appeared, the small pterosaurs all disappeared, and the big ones got REALLY big!