I had a young man come in this afternoon to change his grips that he got a "good deal" on. He allowed me to measure his grips on his clubs and determine the right size for him. He quickly saw that the grips he just got online were definitely too small for his hands and he asked what the difference be. Well, as I showed him by having him grip a club with a grip on it that was too small - the club rotated left (closed) a little when he applied his grip. Then he gripped a club with a grip size the fit him, and the club stayed right where he had it originally - in a square position. I see this more than a student with grips that are too large.
When giving a lesson or club fitting, I always look to see if the club fits the student in all ways, as I know that it will make a difference in performance. People come in many shapes and sizes, and their equipment should fit them. If your are wearing too small or too large a pair of shoes, you will certainly have a more difficult time walking than you would if they fit just right, so do the same for your golf game - get fit and stop making adjustments for improper equipment!
The young man built up his grips to fit - and I'm sure that he will hit the ball much better with grips that are the right size for his hands!