[T]he EU has not had a static membership. Just as America has grown through immigration, the EU has grown substantially through adding more members. Just as immigration has put some strain on the American social fabric, EU expansion has put some strain on the political fabric of the EU take a look at the French reaction to the influx of Poles, or the increasingly unviable CAP. But in both cases, growth has meant greater aggregate economic power, and therefore greater global influence where political institutions have been able to harness that power.
Tyler Cowen weighs in here.