According to a study conducted by Manchester United, playing 4v4 instead of 8v8 yielded:
135% more passes
260% more scoring attempts
500% more goals scored
225% more 1v1 encounters
280% more dribbling skills
European youth soccer has been doing smaller-sided games up to U12/U14 for a long time. U.S. Soccer is now finally implementing the same (click here to see my earlier post on this topic).
[Thank you Peter for bringing this data to my attention.]