Perimeter and Radius
The circle drill offers a great way to practise and conceptualise width of arc. Here are some key points to consider:
Once we allow the arm structure to collapse in the backswing the club swings independently of the body. The club swing is generally too long with the potential for the body to stop turning very early in the backswing. One would then need to find the width again in the downswing and that means extending the arc when it should be narrowing.
We are trying to maintain the width of our arc in the back swing. Note that this means not forcing the arc to be too wide as well as too narrow. Often, I see golfers over extending their arms in the backswing in an aim to maximise arc width. This will pull you off balance, out of anatomical harmony and out of sync.
Shortening of the Backswing
A typical club golfer pattern is to collapse the arms and stop the torso rotation toward the top of the backswing. When this pattern is altered to add width and improved arm structure, the swing will get shorter in length. It is important to accept this know that you will gain speed from other areas. Don't get obsessed with getting the club to parallel or you may find that the arms persist in narrowing the arc.
The general trend of development is that, after some practise, golfers tend to get the hang of structure in the backswing but the follow through is often more elusive. Novices find it hard to focus on the movement through the ball and after impact but this is of uppermost importance. Attaining the width in the follow through will give you improved club delivery through impact. Watch yourself on video and look our for crumpled arms through the ball.
Be aware that the dynamics of the swing mean the downswing arc will be considerably more narrow that the upswing. See more on this one the related video LINK TO POWER APP VIDEO
Images of arcs from 3D
Thinking of the club head on the edge of the circle is a key movement representation that will help you develop an orthodox movement without overthinking. See circle drill intro for more on why this is so effective.