Dragons

[Prince Jon Targaryen]

I was recently thinking about the dragons of old, why they fell, and exactly what their lives where like. Now, we know that damn near every dragon has a specified date of death or at least some description like "died after event x occurred". I was thinking about how comparatively, the dragons Dany has (of Essos) are probably a good deal more robust or healthy than any that might have been hatched from old Targ eggs. This is because these dragons had declined in health and stamina as a result of being bred and raised in the dragon pit. The dragons of Essos, to our knowledge, never received such treatment. But where were dragons like Balerion, Vhagar or Sunfyre kept at King's Landing? Where would Aegon have kept Balerion? These huge dragons needed some place to reside. I doubt any king or prince would want to be very far away from their dragon.

[Lord Varys]

I suggest you read TWoIaF, TPatQ, TRP, and the soon to be published TSotD for that. The old dragons were usually kept on Dragonstone or in the Dragonpit of KL (after Maegor the Cruel built it).

Dany's dragons are not all that large as of yet - I think about the size of Vermax, Arrax, and Tyraxes - and the dragons we see during the Dance are all pretty large (Vhagar, Meleys, Vermithor, Silverwing, and Caraxes being the largest) and healthy before they all get killed off.

[Graydon Hicks]

the dragons began to decline in size, with each successive generation, after the doom of valyria. i have a theory that as the "tide" of magic began to flow out, the spells the valyrians used to keep those volcanoes under control failed, thus the mass eruption, thus the doom. the dragons are creatures of magic, both feeding off, and exuding it. like sponges maybe? as there was less and less magic in the world for the dragons to drink in, each generation came out smaller and smaller, never growing as much as their predecessors. daenerys's dragons are being born into a world were magic was waxing, the "tide" flowing back in, so there is progressively more  and more magic for them to drink is as they age, increasing their growth.