well what else explains it?

Well, endothermy does.

It’s not just size, it’s surface area, and the theropods were pretty slender.

Also, ectotherms still generate internal heat from metabolic processes, moreso for large animals, there’s just no homeostatic control. It’s thought that sauropods, which were particularly bulky, might have been very stable, temperature-wise, but not technically endotherms.

Personally, I suspect that most theropods were endotherms, like their surviving cousins. One problem with the question is that it assumes that endothermy has to apply for an entire group, because the examples found today work that way (if you ignore examples in fish).

Perhaps they were ectotherms and different specializations they had in terms of body shape too assisted their maximal use of the suns heat. Look at the Pelycosaurs for example? the shape of that sail resembling back perhaps? maybe increases the surface are the way leaves are surfaces for photosynthesis. They might have also had highly vascuralized skin, but there are somethings that we just dont know about them to be absolutely sure and deduce the truth from.

haha…. have we thought about the feathered dinosaurs? dorsal feathers on the Sinosauropteryx may be a clue to endothermic donosaurs, some method to perhaps keep heat? or maybe to reflect light if they were ectothermic?