The thing is: too many players don't make the game boring... it just makes it take longer for it to get interesting (unless they're all players of the same level).
I think that limiting the number of starting players might be an option. Another one could be handling it like a tournament: whenever the number of players goes over a certain threshold (30-40?) several ponds will be created each with about 20 players or so. In the end, the winner(s) of each section dispute the final round...
So, for this 249 pond, there would have been 13 sections and the winner of each section would play in a final pond, lasting the wholw thing a maximum of about 40 turns instead of the expected 200 (now less) that happened here...