Physics can be a *****.
Basically, since dragons fly, and they are so large, they need to have some extreme adaptations.
Because physics can be such a problem, the larger you are (in mass) the greater your lift capacity needs to be in relation to your overall volume. For example, if you took a humming bird and put him in a machine that did nothing else but increase his size by ten times, but he is still composed of all of the exact same materials, he would not be able to fly, or atleast, definately not be able to hover and fly with the same agility that hummingbirds normally do. The energy that would be required to beat the larger, more massive wings to hover the larger more massive bird would be far too great for the hummingbird to maintain.
This is because of the nature of physics.
You could drop an ant from the door of an air plane (without a parachute) and he very well could live. That is because the his mass to constituent composition strength is much more advantageous for somethign that weights less than an ounce.
I'm sure you've heard that an ant can lift 10x his own weight. Well, if you grew that ant by 100x and he was the size of a small dog, he wouldn't be able to lift more than his own body weight. In fact, the exoskeletal system would be weak and inefficient, which is why you almost never see arachnids or insects much larger than 1lb in mass.
Any creature larger than 25lbs would need wings designed almost purely for lift based on wingshape. Such a creature would not rely on beating his wings for aerial flight except posibly for getting off the ground. Think of the largest known birds. Many of them rely on spread wing flight and often thermal updrafts to gain and maintain altitude.
And as your mass increase, so does your inefficiency of flight. If a beast weighted more than 1000 pounds, his wingspan would probably have to be over 40 meters in order go gain the lift needed. But the problem is, wings that large would have to weigh 1000lbs all by themselves, or they would simply not have the strength to stay extended, and they would fold under in flight! Just having hallow bones doesn't do it. ....and having cartilagineous exoskeleton to protect you from fire, that is completely out of the question.
So, like I said, physics can be a *****.
Any creature the size of a dragon, in order to fly, would have to have evolved some EXTREME adaptations.