The main problems with a big dragon are, by increasing order of importance:
There can be linear motors (in a broad sense, what muscles are) that are powerful enough per mass or volume. For an unusual evolution that solved the latter problems, this would be straightforward enough. As we'll see later, this is not even the limiting factor in human strength under normal circumstances.
The muscle has to not tear itself and its supports apart (something human muscles will do if you use them at max power). Look up tetanus symptoms - or not, muscles at full power are horrific stuff. But there are materials stronger than human tissues that exist, and that could reinforce dragon tissues. Carbon nanotubes and graphene are rather popular choices, nowadays. Aggregated diamond nanorods can make for pretty nice bones as well.
Muscles need oxygen and sugar (or whatever this organism may use). Those are carried by blood. Sugar level could be increases quite a lot with the right biochemistery, as long as it can metabolize its reserves (probably fat) fast enough to keep up. Oxygen is a bit trickier. Blood could carry more, but oxygen intake is limited by the lungs. They would need not only big ones, but probably something more efficient like a biological supercharger. Waste products like carbon dioxyde need to be removed, but solutions to intake should double as exhaust here.
Probably the most critical, and often considered the biggest limiter in muscle strength for large organism, due to the square-cube law: muscle volume (and theoretically, strength, but also waste heat generation) goes up with the volume, while muscle surface (and its ability to evacuate waste heat) goes up with the surface. So the dragon will need an extremely efficient circulatory system to get rid of heat - you won't get much better than water, so you will have to pump more blood faster and through more vessels. This is both for heat and fuel intake, in fact, as fuel intake is also limited by surface.
Then, this heat will have to go somewhere. The aforementioned supercharger would help, but also the gigantic wings that a dragon needs to fly anyway, and that will make for nice radiators - wings must also grow larger due to the square-cube law. you can even have fun and have the supercharger pointed down, with its scalding hot exhaust a bit similar to a breadth weapon.
Increased perspiration through, especially through the supercharger, may be necessary. There could also be small cold nodules in the muscles to help get rid of heat during a brief, intense effort, though designing such biological system to be small and light enough may be a challenge.
There will have to be a balance between impractically large wings (to better glide) and impractically powerful muscles (for powered flight), depending on how efficient those systems are and how strong dragon tissue is. Anyway, this may either require artificial design or a very unusual evolution, but those may help grow dragons just a bit larger. I suspect the dragon would only be capable of punctual intense effort to take flight, and then mostly glide, like the extinct Quetzalcoatlus mentioned in the answer of Klaus Æ. Mogensen.