I think, though I could be incorrect, that RK's assessment is that this would have a negative impact on the playability of on the PS3 due to crossplay. For instance:
- Imagine the game as is, and cross-play allows a PS3 user to access 100% of all maps available through the system. This means that playing with friends or downloading maps has no limitation.
- If there was a system exclusive world size (and in my experience, the majority of people play the smallest map possible or the largest map possible), this would have a notable impact on cross-play opportunities with that other system, where a non-minimal number of worlds were "incompatible". This would reduce the system specific playability of the PS3 when sharing with people on another console . . . in essence, "now I can't play with all my PS4 friends".
Please note that I understand very little of PS3/PS4 crossplay, so I might not actually understand how this works, but this would be my guess if RK wants to confirm.