How often have you seen someone that swings the club back too far? Perhaps you are even one of those players yourself. The common perception is that this tendency is simply due to an effort to hit the ball farther, and that with discipline the player should be able to swing back shorter into a position where the arms maintain better structure. If we look at it from the perspective of the kinetic chain, we can see other reasons that a player may break down at the top and let then club swing back farther.
In the past I have spoken about leverage being in the ground. Simply put, when stability is lost in the lower body, the upper body will move in a way where there is less strength to support the club. For example, when the lead knee collapses in during the backswing a player may lean towards the target with the upper body which puts more stress on the shoulders to support the club. Without ideal shoulder mobility and stability, the lead arm will break down and the swing becomes long.
If you are looking to shorten your swing up because you have seen it on video, or you can just feel that it is too long and leading to inconsistency then start from the ground up. First see if your knees are over the tongues of the shoes at setup so that you start in a stable position. Then see what happens as you take the club back. Does the lead knee move a lot, perhaps towards the ball or towards the back knee? If so, then you need to improve the lower body movement before you can say it is simply a matter of discipline.
A strong structured position at the top of your swing can be a huge step to swinging more efficiently, being more consistent, and not getting hurt. Start at the ground and work your way up!