I think they explained in a bit more detail somewhere on this site (also a few years ago) -- but basically they're saying Carnivore is better for faster growth in younger, juvenile fish, while Massivore is differently formulated (just slightly) to maintain size and health when adult sized. My young NTT Datnoid goes bonkers for Carnivore; at ~4" he could probably barely fit a Massivore pellet in his mouth, so there's that to consider as well.