When you keep the environment temparature constant, you can make your beasts ectotherms. That would reduce their need for food, since their metabolism would not need to maintain a constant body temperature. They could much more easily survive longer periods of time without food.

If you add a rich vegetation that is capable of maintaining large herbivores (like the brontosaurus), you also give them sufficient food to sustain themselves.

In [this article](https://www.khanacademy.org/science/biology/principles-of-physiology/metabolism-and-thermoregulation/a/metabolic-rate) you will find aood comparison of the metabolisms of endotherms and ectotherms (and also some further references).

Citing from there:

> Endotherms tend to have basal high metabolic rates and high energy needs, thanks to their maintenance of a constant body temperature. Ectotherms of similar size tend to have much lower standard metabolic rates and energy requirements, sometimes 10%, percent or less of those of comparable endotherms

Also, according to the link, the larger size works in favor of your dragon's metabolic needs, since it needs less food per mass:

> Which one has a higher basal metabolic rate: a mouse or an elephant? If we look at the metabolic rate of the entire organism, the elephant is going to win – there is way more metabolizing tissue in an elephant than in a mouse. If we look at per-mass metabolic rate, however, the situation flips. A gram of mouse tissue metabolizes more than 10 times faster than a gram of elephant tissue!