This is because you aren't changing just the exit pupil, when you choose an eyepiece that provides a smaller exit pupil than your eye can fully accomodate, you are also changing the image scale (magnification.) Increasing the image's apparent size generally improves the detail seen, as long as the image doesn't become too dim in the process. For most DSO's an image scale of 1 to 2mm can be quite good for revealing detail. Visual acuity tends to be best in this range. (Some other stuff is going on for planetary, but we are talking about the fuzzy's at the moment.)
Many of the faintest objects will not even be detected until the magnification is in the range provided by 1-2mm exit pupil. Even with the 20" I use 1mm exit pupil when the seeing supports it to bring in the smallest/dimmest galaxies I can detect, but am mostly stuck at 1.4 or 1.8mm exit pupil because of seeing limitations, and sometimes 2.2mm is required or even that is mushy. I will go even higher than this for DSO's if there is something in particular that requires additional scale, such as M87's jet, but the seeing rarely is adequate for such a larger scope.
Where larger exit pupil is helpful is on larger and or lower surface brightness objects. The diffuse outer reaches of galaxies, the weakest nebulosity, the IFN, etc. are best seen with larger exit pupil. There is still an optimum to be found for each object in terms of scale and brightness. Another factor is the use of nebula filters, particularly with the most diffuse emission nebulosity. For the latter the contrast increases greatly because the foreground glow disappears, leaving the glow of the nebula; but it can require a larger exit pupil to provide sufficient surface brightness to see it decently.
Even with small or very small galaxies I often observe the galaxy first at 3.2mm exit pupil (156x) where I am more likely to see low surface brightness aspects in averted vision. Then I employ smaller exit pupils to improve the level of detail seen, but this sometimes results in the loss of the wider extent. And there are a few rather large, low surface brightness dwarf galaxies that I typically observe at 6.2, 5.2, or 4mm. exit pupil, because that is how they are best seen with the aperture.