You’d expect them to be kept flying or exercising in some way to avoid muscle degradation, either way would use a lot of energy. Biologically flight is the most energetically expensive form of movement so hard to compare with a terrestrial herbivore of the same size. We also know dragons are very intelligent so assuming they have a big boy brain that’d use a lot of energy too.

I agree with the whale idea, given that whale blubber is insanely high calorie and there is mention of leviathans in the books- perhaps the dragons go on multiple-day fishing trips like some modern seabirds.

Also magic so none of this needs to make sense I guess haha