You can have two eyepieces that will show more or less the same field of view, but one with a larger AFOV will be shorter focal length and smaller corresponding exit pupil, and magnification. This would result in a darker background.
But you'd be better choosing the exit pupil depending and what and where you observe from. I find that anything much more than about 2mm just sucks in the City sky glow and spoils the view, not so at a dark site
Regarding the ES30/Nagler 31...the field stop isn't necessarily the same as the exposed glass at the bottom of the barrel...