So something that hasn't been mentioned yet; Allosaurus is a Monster in MM2 and it's a 10 HD Huge animal by default. There are advancement rules and the Allosaurus can be "advanced " to Gargantuan by default at HD 21+. Bloodfeast also looks to be around 30~40ft in most scenes long putting him right on that size boarder. 21 HD Gargantuan Dinosaur seems kind of crazy, but its only a CR 10~11 creature by RAW and Tarquin did mention he had a dinosaur training program to make more powerful monsters.

Also Caldar is a Dragon with 8th level spells and looks OLD. Whats more likely? A dragon looking too old for their actual age with a pile of sorcerer levels tacked on, or a specially bred and trained dinosaur being more dangerous than it's stat floor?

Finally if we want to be really geeky here; The default Allosaurus would have to roll a 20 (With bite, let alone secondary attacks) to hit the base Mature Adult Red Dragon. Our level 21 Dino would only need like 16 to hit an Ancient Red Dragon. Dino damage also scales faster than dragon HP due to crossing a size threshold. So Weirdly enough the advanced Allosaurus does better vs an older dragon that the base one against a younger.