Hello,
as other have said the small one is too small, and the medium one stayed in only once, the first time I wore it (at work). I was a bit tense and in a hurry, so I lightly hurt myself and my anus was stressed, staying quite tight to keep it in.
Took my time second time I tried the medium one and it popped right out before I had time to buckle up. The smoothness of the plugs and progressive shape are perfect, but the narrow part is too short for me.