Recently I've run across several major business negotiations in which the number of participants does not seem to bear a logical relationship to the number of stakeholders, the issues under discussion, or even the relative importance of those issues.
Preparation is key to efficient negotiation.
Sending many people to a meeting may indicate that you have not prepared as well as you should. Find out whether your colleagues agree with you, and what their interests are, before you begin negotiating with an outside party. The credibility of the whole side is weakened by internal arguments displayed during the negotiation with another group. Often it is not possible for a single person to represent all the disparate interests of one party to a negotiation. Sometimes, even the most skilled negotiator needs to be able to check with colleagues about how best to handle specific issues.