In a coupled reverse-lens macro setup, stopping down the rear lens can cause mechanical vignetting. The aperture opening becomes a physical restriction in the optical path, so instead of only reducing light, it also cuts off part of the image circle. That makes the field of view appear smaller.

In practice, the rear lens is usually kept wide open, and exposure/depth of field is controlled with the aperture on the reversed front lens instead. If you stop down the rear/zoom lens, you’re more likely to see this tunnel-like effect or edge cutoff.