Larger animals retain heat better. The square-cube law in action. Hence why dragonflies in the lowlands of my country are quite small, but the ones in sub-alpine regions are fricking massive. (Ditto the hares).
Interesting note and a curious thing to apply in my local area. We have lots of cross breed lizards like horned toads and newts and anoles. We love their feeding habits.
Compared to the rainforest lizards, our prairie ones are 4x larger on average. Like dwarf iguanas in some respects with large claws and speed.
I’m also a huge fan of dragonflies here in north Texas - quite common seeing 3-5” long ones zooming about with gorgeous variety of colors. I’ve noticed they appreciate outdoor fans and treat the breeze path as a playground.
The earth was much warmer during the era of dinosaurs, and yet they still grew to such sizes. I'm not sure that body heat retention is a great explanation, especially given the thick fur coats of most modern mammals that were evolved for the cold.
I'm mostly curious how the ecosystem was able to support so many large animals, from the base layer of the food chain up. If there was an ice sheet a mile thick covering all of Canada down to Wisconsin, then most of north America must have been tundra. Were there less trees? Did grasslands thrive across the continent which supported ruminants more, which in turn provided more food for large predators?
I was referencing Bergmann's Rule [0], which really only applies to warm blooded animals but there's also a theory that evolving a larger size helps ectotherms maintain a stable temperature also. [1], and relating to your question on carrying capacity of the food available - the Jarman-Bell Principle [2] that larger herbivores can subsist better on low quality food than smaller herbivores.
And, for interest, Allen's Rule on body appendages for temperature regulation. [3] There's also Cope's Rule - that animals just tend to evolve to get larger, until that's no longer viable. [4]
I can't comment on how valid any of them are, as I'm just an armchair biologist :D
https://en.wikipedia.org/wiki/Square–cube_law