Despite Neopets canon seeming to imply that all Petpets are smaller than Neopets, in-universe I just feel like it makes much more sense that Petpets, like real-world animals, come in a variety of sizes, with some being large enough to be ridden as mounts by Neopets, such as Bikas and Whinnies. (Although I acknowledge that Whinnies in canon are portrayed as small creatures, in "Worth Searching For" I established a headcanon that there is a breed of Whinny that has purposely been made larger to serve as war mounts for the Meridellian and Brightvalian cavalries.) Also Gwyneth is just so much funnier if she's enormous and doesn't quite realize how large she is.
I decided to go in a bit of a different direction with the Airax and make it more of an anchiornithid, because Tyrannia is for paleontology geekery. :) Also I guess Gruslens are some sort of tusked canid?