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I'm inclined to agree with Eric's explanation. If you hold a tube forward of the eye, the illusion disappears as the dark area around the eye disappears.
Irrespective of the illusion ...
Whether the eyepieces produce the same TFOV or not is irrelevant.
The brightness of the field being viewed is directly related to the size of the exit pupil. An eyepiece with 25mm focal length is roughly twice the 13mm eyepiece but the latter will generate about twice the power.
This means the exit pupil of the 25mm will be about twice the diameter of the 13mm. This equates to 2^2=4 times the brightness or an increase in seeing magnitude of 1.5 (as 2.512^1.5=4, 2.512 being Pogson's ratio).
Therefore, you should be able to see stars 1.5 magnitudes lower in the 25mm eyepiece.
Regards, Rob.
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