From my superficial understanding of physics, it seems that large flying creatures like your typical Western dragon wouldn't be possible on Earth, specifically because they would always be too heavy.
Because of the square-cube law and how muscles work (again, superficial knowledge), adding more muscle and bone to a creature increases its weight more than it increases its strength. Smaller creatures have plenty of strength compared to their weight, but as you get bigger, that ratio starts to shrink. A creature can optimize its weight, but eventually there would seem to be a threshold where the amount of strength required matches the amount supplied by muscle and bone, and anything bigger would be unable to move or would collapse under its own weight.
My thought is that reducing the gravity of the planet being considered would reduce the weight per unit of muscle/bone, but it wouldn't reduce the strength per unit. Thus it should push the threshold higher.
First off: does the above make sense? If so, starting from an Earth-like planet as a model, would changing the gravity alone be a viable solution to creating a world with dragon-like flying creatures? Or would the amount of change required be so great as to create other problems or significant differences from Earth?
Keep in mind that the creatures, in addition to carrying their own weight, would also have to carry the weight of their food, as well as some amount of cargo or passenger weight (maybe on the order of 1 to 3 humans, who themselves would weigh less due to this scenario).