The effectiveness of large teams is seriously questioned by the data in this analysis. Large teams (29 people) create around six times as many defects as small teams (three people) and obviously burn through a lot more money. Yet the large team appears to produce about the same amount of output in only an average of 12 days less time....
The "best in class" projects compared to the "worst in class" in this study tracked over the following ranges:
Effort: 15x less effort
Schedule: 1/5 of the duration
Team Size: The "best" teams were much smaller (9% > 10 people, versus 79% > 10 people on the "worst in class" teams).