Three people, including myself, have already shown you that dragons fly by magic.
That this has always been so in TES, it is very old lore.
We have provided links to the relevant articles.
They
do fly assisted by magical means, their wings
are not strong enough to provide enough lift by themselves.
Like with a dragon being understood by a human, or a dragon breathing fire, this is all done by the dragon language. A magical language, that the player uses as dragon shouts.
I suppose it works much like levitation and personally I think its a good thing.
A creature of that mass moving naturally through the air would mean there were no aerial dragon battles at all, or perhaps only one.
An eagle like attack starting at a great height, with a tremendous end speed, then it picks you up and drops you on sharp rocks from a great height.
So for gameplay reasons and interesting battles its nice they can swoop and turn as they do, which they could not when moving naturally for a creature of their mass.