Knowledge@Wharton Research Roundup has some interesting learning about team performance.  Increasing team size improves performance, but reduces team member satisfaction:

When it comes to teams, less is sometimes more. In a recent paper, Wharton management professor Jennifer Mueller found that while larger teams generally are more productive overall than smaller ones, members of the bigger groups were less fruitful individually than their counterparts on the smaller teams.

There are thought to be three major challenges to effectiveness of large teams:

1. Motivation loss: Being one of the members of a large team could reduces the sense of ownership, and hence reduce motivation to perform.
2. Coordination loss: Larger the team, bigger the effort required to coordinate activity.  This would reduce overall efficiency of the team.
3. Relational loss: Large number of people involved prevents members from forming deep relationships.  This lack of networks reduce collaboration and efficiency.

Based on study of 26 teams with 238 team members, the author found significant support for challenges 2 and 3 (but not for 1).  The article of team satisfaction has potential solutions to these challenges.