Carr's Corollary basically states:

The efficiency of a meeting (or group) is inversely proportional to the number of members within.

Consequences of this:

1. Large groups are not efficient
2. Small teams use time more effectively
3. An individual is the most effective unit

Clearly there is a time and place for large meetings. The two main benefits: communication within the group is easier if all hear the message once and it ensures buy-in.

Applying Carr's Corollary, my preference is always to use a small group to define an action plan and bring to a larger group for ratification. Brainstorming in depth items with a large group is almost never good use of time.