But I'll post anyway. On the dietary habits of fictional creatures. :/
I had always thought of dragons as snake-like, having a very slow digestive process, that allowed them to last for a considerable period on one meal. Multiply this by the vast life-span of a dragon, and a hugely sedentary/hibernatory lifestyle, and it only requires one set of pillaging and festing every so often to be more than enough. Add to this the occasional unwise forays of treasure-seekers, and you had a fairly robust diet, for a slow-lived creature.
Balrogs, on the other hand, had to eat plenty, to support those huge, somewhat redundant, wings. The clue is given later in the book - shadows and dust. So think of a Balrog as like a modern Dyson, with a little more flame.
