Woohoo, right up my Alley!
I LOVE this stuff... been reading up on it since elementary school.
As for the whole feathers thing...
I imagine its harder for a smaller body to retain heat. Plus there's the whole warm-blooded issue. Then the issue of metabolic differences. I think the larger dinosaurs (i.e. T-rex, Spinosaurus, Carnotaurus, Giganotosaurus), had bodies like furnaces, making heat all the time. The smaller coelosaurs and raptors had to create an insulating layer to protect them from the cold...
*hides before geeking up the thread*