In one of my previous posts, I blogged about how Sevilla maintained control over the middle against Barcelona and denied any through balls by being compact.
As you can see, even though they are compact, Sevilla are covering an optimal amount of space and controlling the covered space very well. In the red zone they have achieved numerical superiority. The space left on the other side of the pitch is of no worry, because it can be covered by just shifting over to that side as the lengthy diagonal ball is played by Barcelona to switch the play.
Now watch this video.
Against Barcelona earlier this season, in one of the attacking moves led by Andres Iniesta, Atletico Madrid were dragged by the Spaniard into a very small region. Seven Atletico players were dragged into a small block inside their 18-yard box. When defending deep, usually teams look to cover the face of the goal. But Atletico here tried to cover only a portion of the goal that was visible from the left half-space region.
This left a huge amount of gap inside the box and allowed Ivan Rakitic to run into it without anyone marking him or tracking his movements. Over-compactness leads to control over a small area and increases the confusion amongst the players. This move was the result of the run made by Iniesta into the left half-space.