Why are you concerned about the masseter muscle? Unless you look like Buzz Lightyear, most people's masseter muscle isn't that prominent.
As with all muscles, if it's in extremely heavy use, it will hypertrophy (look at the number of male athletes, especially racing car drivers who have prominent ones - they probably clench like crazy when they're driving!)
If you're an average person, eating foods of average hardness, without a night grinding habit, then it's a muscle that's already at normal size, and not likely to shrink much. What's more likely to affect your face is fat deposits from HRT. If you feel yours are particularly prominent, it may be more from lack of fat in your cheeks, then the muscle being excessively large.
I'm not saying HRT won't affect the muscle size at all, but the amount is likely to be so small that it's not going to be particularly noticable.
