There are three problems an animal needs to solve to evolve into being a really, really large flyer
The first is just the general problems with being really big. Insects, for example, fail this sieve even before the flying issue comes into play, because passive respiration imposes a size limit. You need some kind of skeleton that supports your weight, you need a respiratory system that can oxygenate a big body, and you need a food source and an ecosystem that can support megafauna.
The second is the problem of weight. Unsurprisingly, you need to be lightweight enough to fly, and being light enough to fly at big body sizes is challenging. Quetzalcoatlus, the largest of the giant azhdarchid pterosaurs, was as tall as a giraffe but only weighed in at around 250kg. Both pterosaurs and birds have lightweight skeletons riddled with air sacs that make their bodies unusually light.
And then you have the problem of launch. You need to get off the ground somehow, and that generally involves, basically, doing a jump. Getting up to enough initial speed that your wings generate lift. Birds are uniquely disadvantaged in this, because they launch with their feet; so they have this extra set of muscles that are needed for takeoff bursts, but add weight to their frame.
Think about how a plane takes off. You turn on the engines and they start propelling the plane forward, and eventually you gain speed and generate lift. Now imagine you weren't allowed to use the plane's engines, and instead you had a second, separate set of engines just for takeoff. That's basically how birds launch. The need for strong enough leg muscles limits their size.
This is the core difference that allowed azhdarchids to grow so huge. They walked on all fours, using their wings/arms as forelimbs, and they would 'quad launch'. So their launch muscles were also their flight muscles, enabling them to launch even though they're huge.
These biomechanical limitations apply even to synthetic organisms, which is why when the machines got ahold of the gene editing vats, they designed the terrortross with that weird quad hybrid-wing body plan that doesn't much resemble a bird. Now strap your helmet on and get back on the flak artillery, rookie, you know they only attack at night.