What is likely happening is you are receiving the Canada Worker's Benefit (CWB). There is a point where your income reaches that is like a "sweet spot" and you are receiving the maximum amount of this credit.

When you start adding more deductions to our Self Employed income, the CWB benefit is lowered and therefore you get a smaller refund.  

I originally replied to the email, but that reply was un-deliverable.  My reply was:

Thank you,