As you can see from the nginx documentation, what you are asking is simply not possible.
This is a completely normal byproduct of the monit program monitoring various services on the computer. It is not ideal to change the rate at which monit checks, nor is it ideal to disable the checking on nginx, as it can lead to situations where you'll need to ssh into your box and restart nginx manually.
As has been said, and without sounding rude, it seems unnecessary that you're concerned with this. Can you explain how come you want to reduce size of the logfile?