How to Get more Distance without the Overswing
The length of the swing does not determine if it's an overswing or not. Phil Mickelson has a big backswing, but it's not an overswing.
As long as you can keep your balance, you don't have to worry about fixing your overswing.
However, most average players overswing in an effort to compensate for lack of distance but end up losing even more distance.
Left Arm at the Top
What you want to avoid when trying to get more distance off the tee is too much bend in your left arm at the top of swing.
If you bend your left arm too much at the top, you may feel like your swing gets bigger. However, this will cause your arm and the club to get stuck behind you on the downswing.
You will tend to block the shot as a result.
But I'm not saying you should keep your left arm straight during the backswing.
In fact, most tour pros have some bend in their left arm on the backswing. So it's ok to have some bend in your left arm.
But you don't want to have too much bend in your left arm at the top.
How can You Take a Bigger Swing?
To make a bigger backswing, focus on turning your shoulders or body more instead of trying to swing your arms.
The length of the swing is determined by the amount of turn you have with your shoulders or your body.
To turn your shoulders more, you need to turn your hips as well. Some teachers might tell you to restrict your turn of hips. But hips and shoulders are connected. You can't turn your shoulders without turning your hips.
Stand up and try to turn your shoulders without moving your hips. You really can't turn much. But if you allow your hips to turn, you will be able to make much bigger turn with your shoulders.
To turn your shoulders more than normal, focus on turning your hips. You need to turn not slide.
Also, flaring out your right toe a bit more than normal will let you turn more on the backswing.
Another way to turn more is to pull back your back foot a bit at address. Make sure you don't close your shoulders with it.
This will let you make a bigger turn even if you don't have a supple body.