Works in the same way being big on a small mount does, change your characters form via deviate fish or something else, mount up on one of the basic kodos that used to be 60% speed and then turn of the model changing buff.
I don't know if it works for any other classes / mounts.
Im a tauren and when I do this on a basic kodo I become a small tauren on a big kodo.
Il try to upload a screeny