Exit pupil or ' eye box ' is governed largely by the front objective size vs magnification . Having said that ,
pone of my all time favourites is the Hensoldt ZF 4 -16 .
Yeah, the size of the exit pupil (among other things) contributes to how big or small the eye box is. Another factor is your own eye's pupil size at any given time (it changes depending on viewing conditions and one's age).
From:
https://en.wikipedia.org/wiki/Eye_relief
"The eye relief property should not be confused with the
exit pupil width of an instrument: that is best described as the
width of the cone of light that is available to the viewer at the exact
eye relief distance. An exit pupil larger than the observer's pupil wastes some light, but allows for some fumbling in side-to-side movement without vignetting or clipping. Conversely, an exit pupil smaller than the eye's pupil will have all of its available light used, but since it cannot tolerate much side-to-side error in eye alignment, will often result in a vignetted or clipped image.
The exit pupil width of say, a binocular, can be calculated as the objective diameter divided by the magnification, and gives the width of the exit cone of light in the same dimensions as the objective. For example, a 10 × 42 binocular has a 4.2 mm wide exit cone, and fairly comfortable for general use, whereas doubling the magnification with a zoom feature to 20 × results in a much more critical 2.1 mm exit cone."
From:
https://en.wikipedia.org/wiki/Exit_pupil
"A set of 7×50
binoculars has an exit pupil just over 7.1 mm, which corresponds to the average pupil size of a youthful dark-adapted human eye in circumstances with no extraneous light. The emergent light at the eyepiece then fills the eye's pupil, meaning no loss of
brightness at night due to using such binoculars (assuming perfect
transmission). In daylight, when the eye's pupil is only 4 mm in diameter, over half the light will be blocked by the iris and will not reach the
retina. However, the loss of light in the daytime is generally not significant since there is so much light to start with. By contrast, 8×32 binoculars, often sold with emphasis on their compactness, have an exit pupil of only 4 mm. That is sufficient to fill a typical daytime eye pupil, making these binoculars better suited to daytime than night-time use. The maximum pupil size of a human eye is typically 5–9 mm for individuals below 25 years old, and decreases slowly with age"