Working larger groups do force a dog to slow down otherwise they end up pushing up into the middle of the group. Large groups also make a dog flop side to side which is required to keep all of the group moving but can create the habit of flopping even when it is not needed on smaller groups.
We like working all different sized groups so that our dogs and we know the differences in how they move. 3-5 sheep can generally be worked from behind all the sheep. Much larger groups will often require the dog to move up the sides of the group to control the movement of the lead sheep which they cannot control from behind the group.
It's fun (educational) to move a large group of sheep across a stream (sheep don't liek to get their feet wet). The group goes up to the bank and the lead sheep stop; now the dog needs to go the group to the lead sheep to get one or more to cross teh stream. The lead sheep will not feel the dog behind the large group.