It is true of the number of people in a meeting. It is equally true of the number of meetings in your day. And of the number of items on the agenda.
This is yet another phenomenon from Behavioural Economics. We have to be good jugglers (and women seem famously to be particularly good at it). But juggling comes a poor second to concentration on one meeting and one agenda item. It is hard to give your best in the 11.00 meeting, when you’ve scarcely recovered from the appalling 9.30. Not to mention the fact that you are supposed to making the controversial case for a 30% increase in spend in the 12.00-14.00 with the inedible sandwiches. Equally six agenda points is a lot to cover in an hour. The Mote agenda (we call it a motion) scarcely ever has more than one item to discuss. And three or four people in a Mote will use time much better than thirteen or fourteen in a big meeting.