1. The Risky Shift

People put less effort in a) when they think other people in the group are putting in less effort, and b) when they think other people will work harder to accommodate.
Members of small teams are less likely to think they can get away with shirking if they can’t find other shirkers.

2. Easier to talk to everyone

Once you get above 10 people in your team it becomes logistically difficult for team members to establish feelings of trust, mutual accountability, and cohesiveness. Without these, constructive interaction is difficult.

3. Sheep herding

Small teams spend less time coordinating the efforts of team members. Think about the last time you had to book a meeting room for your entire team, How much time was wasted finding avabilable time and a room.

4. Chameleon effect

With large teams, it’s very easy for people to exclude themselves from participating in group activities and discussions. A high performing team cannot improve unless it hears from all of its members.

5. Satisfaction and Recognition

Small teams are more satisfying, fact.
We all know we need to celebrate our wins (if it’s not worth celebrating was it worth doing?). In large teams it becomes difficult to visualise and add meaning to one person’s contribution.
With a small team, it’s easy to give praise to good work, which, in turn encourages good work.

6. Knowledge sharing

With large teams people are more likely to specialise in their own areas.  For example, one developer only works only on UI. This not only creates hand-offs and gates between team members, but also reduces the amount of knowledge sharing between team members had they been working beyond specific job roles.