Volume to surface area ratio.
A=4 pi r2
V= (4/3) pi r3
V/A= (4/3) pi r3 / 4 pi r2
V/A=r/3
IOW big things retain heat better than small things all other factors being the same.
from the article posted:
"Dinosaurs were long considered to be cold-blooded reptiles.
More recently, some researchers have proposed that the extinct creatures actively regulated their body temperature like mammals.
A study in the journal Plos Biology now suggests this is not the case, but that bigger dinosaurs may have lost heat so slowly that they stayed warm anyway.
Reptiles tend to be cold-blooded ectotherms, whose internal body temperature is dependent on the outside environment. For example, lizards and snakes will sun themselves on rocks in order to heat themselves up.
The question of whether dinosaurs were warm-blooded or cold-blooded just doesn't have a simple answer
Dr Angela Milner, Natural History Museum
Birds and mammals, on the other hand, tend to be warm-blooded endotherms. They can regulate their internal body temperature regardless of external influence.
Their body temperatures tend to be more constant than those of reptiles and higher than the outside environment."
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So, the large dinosaurs obviously were not reptiles, correct?