Consider the following thought experiment:
Take a refractor, it doesn't matter what focal ratio but say the aperture is 100mm (just so the aperture is bigger than a 46mm field stop). For simplicity eliminate the diagonal, so that the eyepiece is inserted directly into the focuser, "pirate scope" style. Let the eyepiece be a 2" model with the max field stop, so 46mm inside diameter for the field stop/barrel.
Now imagine shrinking yourself down so you are a small fraction of a millimeter tall, and can walk around inside the OTA.
Stand at the left edge of the field stop and look towards the objective. How much of the objective can you see? Not all of it, cause the eyepiece barrel, which extends in front of you creates a "horizon". Roughly speaking, since the horizon is 23mm to the left of the optical axis, you can't see any part of the objective that lies more than 23mm to the left of the optical axis. So, there is a crescent shaped portion of the objective that cannot contribute light to the edge of the field at your position.
Now put a 41mm diameter eyepiece stop, or a 38mm adapter somewhere out in front of you. These thing necessarily "raise" the horizon and block an additional crescent shaped section of the objective, increasing the degree of under illumination at the field edge.
If the restriction is severe enough, you won't be able to see the objective at all, meaning the degree of under illumination at the edge has reached 100%.
Now begin floating to your right, towards the optical axis. Slowly more and more of the objective becomes visible over the horizon, however you may find that before the entire objective is visible, the end of the focus tube appears, and is now the thing creating a horizon and blocking a smaller section of the objective. You are now in an annulus of field that is unaffected by the barrel/other local restrictions, but is still under illuminated due to the focus tube. Moving further to the right, at some point the entire objective becomes visible. You are now at the edge of the fully illuminated portion of the field.
In this example the focus tube (or baffle in an sct) is what limits the size of the FULLY illuminated field, because it forms the last horizon, when moving towards the optical axis, before the entire objective becomes visible. However, the 38mm - 46mm restrictions closer to the field stop cause additional under illumination at the edge beyond what the focus tube would cause by itself because they form the relevant horizon when your standing at the field edge. So, removing anything smaller than 46mm gives you some additional light in an outer annulus of field. May not be detectable to the eye, but I think it's interesting.
Using the geometry of my 10" meade, the barrel of a 2" eyepiece allows over 75% of the secondary mirror to be visible from the edge of a 46mm field stop. With a 38mm adapter in place, less than half the secondary is visible from the same position.
I first did this by drawing the rays, but thought the shrinking idea would be easier to explain than the drawings. Let me know if this makes sense...