This speculation is partly based on the Google, Bing, and Babylon translations of "Randpupille," which is "edge of (the) pupil," or "pupil edge," and Dale's reference in post #22 to the "edge bundle." There he intimates that it is a measurable property of the instrument, and that an increase in FOV sacrifices viewing comfort. To me this doesn't add up to Holger's interpretation of off-axis vignetting, or Henry's SAXP explanation (no offense, guys).
So what could it be? Although this is nothing new, paraphrasing from Freeman and Hull's Optics (pg. 535), it is notable that with a photopic eye pupil (e.g. 2.5-3mm), and a large XP, e.g., (5mm-6mm) the outer zones of an instrument's exit pupil and associated aberrations play no role in the formation of the instrument-eye image on the retina. In other words, this donut-shaped area at the outer edge of the XP prevents a large percentage of the instrument's aberrations that are contained in the outer "light bundle" from degrading the retinal image. Hence, it may be the co-called "randpupille" and a direct correlate of viewing comfort.
Increasing the field of view doesn't change the XP, so Dale may have meant that the effectiveness of the randpupille is diminished with increasing field, not its own size. That would make sense since as FOV increases all the oblique aberrations increase exponentially with field angle. Hence, for a given randpupille, increasing the field significantly increases the aberrations and reduces relative image quality (or comfort). I would guess that to compensate one would need to increase the aperture or decrease the magnification, i.e., enlarge the XP.
If true, this notion fits rather well with binocular configurations, large and small, that were historically thought to be "comfortable." Moreover, if the metric were something like the aberration density contained within the randpupille annulus, expressed as a percentage of total instrument aberration, this might have evolved as a binocular design objective. Of course, using it would also require a standard observer operating under standard lighting conditions.
Ed