The radius of a golf swing can change due to a number of swing flaws. But the bottom line is that it's usually a BAD thing when the radius lengthens or shortens in relation to the swing center that was established at address. After all, the club face is only a few inches high and the sweetspot is very small. If the radius lengthens, fat shots and/or deep divots can result. If it shortens, thin and topped shots can result. I've gone back and forth with first thin, then fat, and now thin again. Not fun, especially when that thin shot goes straight. Difficult lies can aggravate changing radius problems.
So what are the swing flaws that I've seen in myself?
I think the top one for me has been flipping the wrists at impact, such that the left wrist cups instead of bows. This is an unconscious reflex of the beginning golfer, who deduces that this motion will help the ball into the air. When the ball is caught flush, it will increase the loft of the club and make for a higher shot (basically turning a 7-iron into an 8 or 9-iron). Distance suffers as a result. But when the ball isn't caught flush, a thin or topped shot results. This, I believe, is what's currently causing me the most trouble. The cure is to work on the opposite, correct feel--a flat left wrist at impact. Think of turning the knuckles down at impact (or revving a motorcycle throttle). This delofts and closes the club face, turning a 7-iron into a 5-iron and resulting in a straighter, longer, lower trajectory ball flight. The right wrist, analogously, stays cupped or bent back more through impact, so one can also try bending the right wrist back more at impact to counter the instinct to flip the right wrist forward. Doing this also increases lag in the downswing. Hank Haney says that this needs to be a conscious swing thought for most golfers, though others feel it happens naturally. It makes sense, however, that if flipping is part of my muscle memory that I'll have to consciously work to make the correct move.
Maintaining the posture or "spine angle" is an oft-cited fundamental. Basically, losing one's posture changes the radius created at address. A basic example is losing the bend from the waist, which can occur in the backswing or the downswing. One of my bad habits is that I occasionally raise up slightly from the waist on the backswing. It's possible to compensate for this one by returning to the bend established at addressed…not easy, but possible.
The knees also play a role in a proper return to the ball. Many beginning golfers like me think the right leg should be straight on the backswing and the left leg straight at impact. Not true (unless you're after the S&T swing). I have a tendency to over straighten the right knee on the backswing , but I usually compensate by rebending the knee before impact. The left knee should also stay slightly bent in the downswing and only fully straighten into the followthrough.
Finally, the elbows. Chicken-winging can result for a variety of reasons and can occur on the backswing or downswing. I do this less now, but I need to work on extending more in the through swing and followthrough.