The memory used by the SQL Monitor base service depends on the current workload.
Typical usage is around 400MB for ten or so servers, but what you're seeing is not unusual for very high loads.
The logs created by SQL Monitor may highlight performance issues that lead to increased memory usage.
- How many servers/instances are you monitoring?
- From task manager, how many threads is the base service using?
- Is there anything interesting in the application logs (ProgramData\\Red Gate\\Logs...)? (you can email these to me if necessary - firstname.lastname@example.org)