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My theory goes something like that:
The image available at the focal plane of the scope is defined by the mirror or lens (say fl.1830mm F4.5).
The EP is looking at the image available at the focal plane, a short EP looks at a smaller area of the image at the apparent Field of View, a longer EP a larger area.
The brightness of an object at the focal plane is constant but the way the eye is seeing it depens soley on the EP.
For example, with a 26 mm x 68deg. EP the circle of the visible image is 35mm.
A 13mm x 68deg EP will show 17.5mm of the image.
The object will appear twice the size but only 1/4 as bright.
The rest is in the quality and design of the EP.
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