Will Masseter Muscles Shrink over Time, if I Don't Use Them a Lot?

Some patients have a genetically larger than normal masseter muscle.  Some patients masseter muscle is increased by over activity.  In theory, if you stopped using your masseter muscle and it was not genetically enlarged, you may expect to see some reduction.  Unfortunately, in many patients with enlarged masseter muscles, the masseter is contracted involuntarily, especially at night.  There is no harm in trying to atrophy your masseter muscles yourself, it is just very unlikely that they will reduce in size.   Usually 2 sessions of botox will reduce the masseter and break the habit of night time clenching.

Generally speaking, muscles that are not being used atrophy. That having been said, simply stopping gum chewing will unlikely produce noticeable volume loss since you will continue to use the muscles to eat. Also, there is always some resting tone meaning that even when the muscle is not being used, it is under minimal contraction. Ultimately, it would not hurt to refrain from gum chewing to see how much improvement you get if any at all. If that doesn’t work you can consider botox. Botox is very safe and only minimally uncomfortable to have injected, but most importantly it works reliably.
Also, make sure that your issue is in fact that your masseter muscles are enlarged and not that you have prominent buccal fat pads. If the problem is the latter one, botox will not help and you may instead benefit from a procedure to reduce those fat pads. Good luck.

I have used Botox often especially in Asians who want their masseter muscle reduced. 20 units each side would be perfect to get masseter reduction

Like any muscle, if you don't use it, it will atrophy and shrink. But you use that muscle when you chew, talk, etc. so just stopping chewing gum may not be enough for you. It will take months and months if not longer for the muscle to astrophy back to its "normal" size.

"This answer has been solicited without seeing this patient and cannot be held as true medical advice, but only opinion. Seek in-person treatment with a trained medical professional for appropriate care."