Another take from televue:
http://www.televue.com/engine/page.asp?ID=141
The biggest factor it seems, may be the size of the central obstruction. That makes sense.
The following is copied from TeleVue:
Myth #1: A 7-mm exit pupil gives the lowest useful magnification.
Not so! With a refractor there is no limit on the size of the useful exit pupil. Use whatever is necessary to get the field you need to frame the subject. A reflector's low-power limit is reached when the black spot in the exit pupil (caused by the secondary obstruction) becomes obtrusive.
While a 7-mm exit pupil, by matching that of the eye, does give the brightest views of deep-sky objects, it does not necessarily give the best ones. Higher magnifications, despite their smaller exit pupils, will reveal more details, maintain contrast, show fainter stars, and help bypass defects in the eye itself.
Myth #2: Exit pupils larger than 7 mm waste light and resolution.
With refractors larger pupils do waste aperture. But the magnification is so low that the wasted aperture is of little concern: both image brightness and resolution are as great as possible at that magnification. With reflectors, however, larger pupils do waste light, but primarily because the black spot in the pupil caused by the secondary obstruction becomes larger. Both light loss and field shadowing occur with reflectors, but as with refractors there is no resolution loss because of the low power.
