This is a well known problem in flight of dragons. A large reptile would need far more wing surface and power than some human-sized angel. There are several solutions. One simply defines dragons as magical creatures, and therefore science doesn’t apply to them. Another method gives dragons control over gravity. I even read about beasts that have control over time.
They would need some unknown kind of metabolism that is more efficient than what we currently understand. Seems like I read somewhere that pronghorn antelope have something unusual going on in their metabolism. This enables them run at top speed for much longer than what they should be capable of.