I know this is an old thread but I was wondering myself why Factions pets don't grow. It kind of sucks that I level up my white tiger only to have it the same size as when I first got it. I asked Gaile last night why they don't grow and she claimed that they DO grow. My tiger is a hearty one. Is there something I didn't do correctly? It just seems odd that they skipped pet growth in Factions when it started in Prophecies and continued on in Nightfall. For me the larger pet shows the accomplishment much like ascended armor or titles.
Why? Nobody here can really answer that because we aren't developers. Maybe they ran out of time at release, maybe they forgot, maybe they decided it was a bad idea.