The reasoning behind cramming your own box is that it restricts space for the opposition and should make it easier to defend.
I’ve never agreed with this. A crowded box eliminates the goalkeeper’s ability to do anything other than stay on his line, while the defending players have to concentrate on a higher number of attacking ones, leading to a higher chance that someone will lose his marker or that players will get mixed up. Then, if you do win the ball in and clear it, you have nobody to contest the clearance so it’ll come straight back in to that same crowded box that’s if anything more disordered. We conceded a fair number in that manner last season and the season before.
I would suggest leaving 3 men up, one on either flank and the other in the centre, to force the opposition to bring players back. This then effectively removes 6 players from the box, and brings the keeper back into play as someone who can also help defend the area. You simplify the task for your own defending players as there are fewer attackers to worry about and people’s roles are themselves less complicated (no defending subsection 2B of the 6 yard box). Moreover you’re left with 8 players in the box defending against just 6, as opposed to 11 against 9. The net balance hasn’t changed.
If you get the ball clear you then also have a fair chance of setting up a counter or at least preventing the ball coming right in.
Better for both defending the set piece and hitting on the counter, I rest my case.