I think sweet spot often refers to how big the sharp part of the image is. The Swaro EL's excel in that. Both from side to side (edge to edge) and from near to far (depth of field).
What you are describing looks more like the ease of eyeplacement to take in the (whole) view. That improves with a larger exit pupil of the instrument and is best in the 6-7mm exit pupil range but is also influenced by eyepiece design. The Germans have a unique word for this: "Einblickverhalten". Which might be translated into: " the ease to take the views in" or "behavior of the views in how they present themselves to the observer".
With your example of darkening when looking at objects at closer distances that can be overcome by adjusting the inter pupillary distance is likely a matter of the exit pupil of the instrument no longer lining up perfectly with the entrance pupils of your eyes. This is easiest with the largest exit pupils fitting in the smallest entrance pupils, hence the 7x50 binoculars being a favorite for marine use.
Another thing to consider is that to an extend, all binoculars have uneven illumination of the field. Illumination is best in the center of the field and reduces towards the edge of the field and is caused by vignetting.
Random black-outs dancing through the field of view and letting large parts of the views disappear, also known as kidney beans, originate in spherical aberration of the exit pupil. This phenomenon, which plagued the larger of the TeleVue Nagler type 1 eyepieces, is described in more detail in this great optical reference work.
These are but a few of many characteristics of an instrument considered by manufacturers when designing and manufacturing their binoculars and influence size, shape, weight and cost of that instrument.
Hope this helps.