In versions prior to 14w26c, mob spawners would check for the target mob in a cuboid with X and Z lengths 4*SpawnRange+1. For example, a spawner with default settings (SpawnRange = 4) would stop spawning when there are more than 6 of the target entity in a 17x9x17 area around the spawner. In 14w26c, this has changed to 2*SpawnRange+1, i.e. it now checks a 9x9x9 area around a spawner with default range.

It may be this change in behavior was intended, but as it wasn't announced or part of MC-4855 I'll report it. The change certainly would make spawner-based mob farms easier.