Have worn dearfoams for years and I have a slim foot - always get the medium - this has a lot of excess shoe near the toe and at the rear - gams on me near area of the stretch side panels. In the past, the slippers did not have such thick foam - I will keep them since they are OK, just not as good as in the past.
In contrast - bought same pair for my mother in the large which have to go back - she has a wider foot than I but still only about average width - they are too tight for her. Again, I think the problem is due to the excess foam padding - I bought her about six pairs of dearfoams in the past a few years ago and all were perfect. Perhaps buyers should try a different style than this one.