exit pupil = (eyepiece focal length) / (scope focal ratio)
So for a 42mm eyepiece at f5 , exit pupil = 42/5 = 8.4mm
Another way to calculate the same thing is (aperture) / magnification.
mag = 1250/42 = 29.76 , 250 / 29.76 = 8.4
Nobodies pupils will dilate enough to allow an 8.4mm exit pupil and the other factor is that the size of the shadow of the scopes secondary is also getting big which hampers viewing.
Try looking through your scope with your lowest mag eyepiece in daylight to see what I mean.
In general terms the best views come with exit pupil size less than 6mm or so depending how dark your observing location is. Go bigger and the sky background washes out and becomes grey instead of black. Furthermore, your eye is vignetting the light and you are effectively loosing telescope aperture.
This is why we look for wide field eyepieces especially at longer focal lengths to keep exit pupil size down.