Nope. The bigger dragons were Thaumivore's (as in they ate/used magic). Because they ate magic they could do stuff such as fly while weighing tonnes and not have all the problems that Swamp dragons do. However when magic became more rare they started becoming rare and some went to "another place" where there was magic.

The Swamp Dragons though remained behind on the Discworld and because there was no magic had to face all the problems that they could use magic to solve before.

Errol was either in-bred or a mutant or just a very smart dragon who figured out that fire from mouth means burnt food but fire from a** means jet propulsion.