Hi Ali,
First of all - sorry for the slow response. I somehow missed this message.
The MultiSocialLeaderboard call is normally used to create Candy Crush-style maps - where you would normally show just the top player on each node of a map.
It's otherwise a pretty expensive call (as we're filtering/joining your friends list with multiple leaderboard lists) - so thus we restrict things to keep things fast and server utilization reasonable.
Do you have a use case that requires the larger leaderboard sizes - or maybe you were mistaking the intention of this call with another?
Anyway, hope that perspective helps!
Cheers,
Paul.