If you really want to know what's going on, put the dimensions and specs into Stellafane's "Newt for the web."
https://stellafane.org/tm/newt-web/newt-web.html
The diagonal size, tube dimensions, back focus and the focusser dimensions all contribute to field illumination. Increasing the diagonal alone may not increase field illumination. This web application does a detailed ray trace.
Diagonal wavefront accuracy
Light reflecting off a 45 degree surface doubles the diagonal's wavefront error. If you have a 1/4 wave primary mirror you probably want at least a 1/8 or 1/10 wave diagonal to match. I think Skywatcher mirrors are spec around 1/10 wave?? Spec and your mirrors actual wavefront error can be different. If 1/10 wave, your diagonal should be 1/20. Make sure both measurements use the same method eg peak to valley, rms so that the accuracies are comparable.
Joe