Reversed Lens – Changing the aperture changes the field of view

I have a reverse lens macro setup, with a 35mm inverted lens attached to an 80-200 zoom lens via an adapter. I have the following problem: Changing the aperture setting on the zoom lens changes the field of view in my system.

I'm not sure why the aperture ring changes the field stop. Can someone help me explain this?

Thank you so much!