Have you ever been on a project that is falling behind, so someone up the food chain decides you need more people and then you fall even farther behind?
One reason for this has to do with communications. The more people you have on a project, the more communications channels you have. Two people, one channel...10 people, 45 channels. Agile works because you keep the teams small and keep everyone in the same room (ideally), minimizing the amount of communications you need.
Joel Spolsky recently wrote an article in Inc on this topic. He mentions the team responsible for the main menu for Vista at Microsoft, 43 people in all, or 903 communcations channels. In a year, only 200 lines of code were written because so much time was spent coordinating.
So what do you do? Think more about your communications. Don't invite people to meetings who don't need to be there. Don't send email to large distribution lists. Take a little more time planning your communications instead of blasting it out to people just to keep them in the loop.