It would make sense. Dragons get more powerful as they age, and seeing as Bahamut and Tiamat made some sort of proto-universe (The First World) which shattered when other gods tried to colonize it, resulting in all the different DnD worlds and timelines (as well as the two of them becoming eternal enemies, so I'd bet my boots that Tiamat wanted to kill anyone that tried to colonize her world, while Bahamut sided with the colonizers and protected them. Of course the colonizers ended up capturing Tiamat and tortured her until she had five split heads, and actually did overload and shatter the First World, also killing Sardior in the process, so perhaps it's not so hard to see how Tiamat went from creating a world to being the biggest, baddest and most far-fallen villain of all time)...

Well, my point is they must be really freaking old. Perhaps very little of their power has to do with being divine, and it's more just that they're nearly as old as the universe. The divinity just means they can't die of old age or be permanently killed, so they can (slowly) get more powerful forever.