Flapping flight bumps into a size limit after a while because of wingbeat frequency.
My knowledge on birds is rather limited, so here's how I interpreted that:
A longer wing is a longer lever -> Longer lever is harder to move back and forth and also take more time -> Muscle power output can scale 1:1 with mass -> but we also have to account for the longer time-> gravitational constant is: an object is accelerated towards the gravitational center of the planet by 9,81 meters per second
How can I preserve wingbeat frequency while keeping dragons aloft and not completely messing the aspect ratio over?