You're right that looking at only the size of an animal doesn't tell us much about its thermoregulation. Cold blooded animals (i.e. ectotherms) come in all sizes from massive sharks to small reptiles and of course insects. Warm blooded animals (i.e. endotherms) are a little more limited at the small end of the spectrum because of the scaling of surface area to volume (Y = aX^(2/3)) making heating proportionately more costly for smaller animals.

Endothermy for small mammals can be too costly because their high surface area to volume ratio causes proportionately more heat loss than if they were 10x larger. This is related to another point brought up here at the opposite end of the size spectrum. Large endothermic animals like elephants are more at risk of overheating because the dissipation of heat is limited by their surface area to volume ratio being much lower. All animals have metabolic rates that are appropriately scaled for their body size or they wouldn't be here!

I think it's also important to include here the spectrum of heterothermy to homeothermy as a concept. Heterotherms maintain a 'wide' range of body temperatures and include some endotherms (e.g. squirrels that decrease their body temperatures to hibernate) and a lot of ectotherms (e.g. reptiles living through day-night swings in ambient temperature in the dessert). Whereas, homeotherms maintain a more narrow range of body temperatures but also include both endotherms (e.g. humans, only experience very small changes in body temperature) and ectotherms (e.g. many fishes are restricted to narrow temperature bands). But what really is the difference in heterotherms and homeotherms? It comes down to the sum of all biochemical processes in the body. In a homeotherm, all of the enzymes, signalling, membranes etc. are optimized to function at a set temperature (i.e. specialized) whereas heterotherms have cell components that can continue working across a broad range of temperatures (i.e. generalist). This kind of segues into what I think is beneath your question which is why endotherm when ectotherm do job?

The clearest answer is that endothermy allows animals to occupy different thermal niches than ectotherms. If it's beneficial to spend a little extra energy maintaining a constant body temperature with specialized cell components that work faster (because warmer temperatures mean faster reactions) to occupy different niches than ectotherms. And on the flip side ectotherms may be better suited to handle disruptions in food availability over time because they are not burdened with the metabolic furnace bill.

There is of course a lot more to this, but it is a really interesting evolutionary question. My understanding is more from the physiology side of things, hence the answer.

edit: removed ~ in front of 2/3 because it looks like '-' in superscript