I just got his private message and replied, but since this thread is in a public forum I will tell you approx. what my answer was (if you are curious :p):
Basically, pupils of different sizes are very common. They are not generally a concern as long as the pupils react normally (to direct light, accommodation, and consensual responses). Also, is the size difference something new, or has it been there for a long time and not noticed? However, there are some conditions that need to be ruled out, and can only be done so effectively with an exam.
One big issue for us with MS is the ever-loveable optic neuritis
With ON you can get an afferent pupillary defect (APD). This is a pupil that dows not react properly to light.