The issue may not necessarily be the RMR so much as it is your eyes.
When you are in dim light your iris opens up, allowing more light in, but in doing this, just like a camera lens, you loose depth of field focus and allow any inconsistencies in the lens of your eye to cause the smearing or star burst of the dot. In bright light, your iris is much smaller therefore you are looking through a much smaller part of the lens, right in the center.
Red is particularly bad about this because it is at the end of the color spectrum and has more diffraction than any of the other colors.
If you have had Lasik, like me, you are specifically susceptible to this issue. In low light I have some serious issues with my red dot looking like a red smear rather than a dot.