I think size probably was a factor for the otter, but not because PZ is adverse to small species. I think because the aquatic pack is effectively introducing a new game mechanic (diving), they probably wanted larger specimens of iconic aquatic species to better show it off. So we got a large penguin (king) and a large otter (giant), but we already had large crocodilians so they switched it up with caiman for the small species of the pack. This is also probably why the salty alone got the diving treatment (for now). But as others have mentioned, the aquatic pack opens up the features for future packs, so the smaller species will probably get their day in the sun down the line.
Also, this pack looks amazing. Thanks Frontier!