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What happens is that the aperture stop becomes the pupil of your eye, and the effective aperture of your scope is reduced to the eye pupil x the magnification. It's much the same as putting a mask on the front of the scope that reduces the clear aperture.
On reflectors the obstruction ratio of the secondary increases (in percentage terms) and you may tend to see a large out of focus black blob swimming in the field of view, which is your secondary. It is particularly bad with short focal ratio Newtonians (f/5 or less) and also SCT's where the central obstructions can be 30-33% of the original aperture. The "black blob" effect is also made much worse if the eyepiece has a lot of spherical aberration at the exit pupil - which some Naglers do suffer from, mainly the early ones. Also known as the "kidney bean" effect.
Which is another reason to opt for a reflector with a smaller secondary, even if this means f/7 or more.
With a refractor you're just losing effective aperture, there should be no "black blob" effect.
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