The reason why we decided on having a limit on team members is for teams to choose their members carefully and to stop abuse of unbalanced teams and undistributed resources.
For example, if we don't limit members, a team could have 5 programmers, 2 designers, and end up doing a small game that could've been done with, lessay, two programmers. Which means 3 programmers could've joined other teams.
That, and we had a scenario in the last game jam where a team of 6 or 7 members (Can't remember correctly) failed to present their game because their lead programmer had 2 or 3 beginner programmers to teach them how to use Unity3D. Because he got busy and couldn't really stay for long, they couldn't finish a presentable demo of the game.
We don't want that to happen again, so we have to limit team members. However, if we find that 4 members weren't enough and there are requests from teams to raise the bar, the bar will rise.
I hope you understand the situation.