The mediums are a little narrower, so slightly trickier to stuff. Up until just recently I preferred mediums on Lili
(she's pretty slim, 24 lbs at 2 yrs) and they still fit her fine but are starting to get a bit just in the leg. She wears M high rises on the 2nd popper (waist and thigh, though I should probably go to the 1st one!) and L standard rises on the 3rd popper.
I didn't like how the wings on the larges rode up in the front at first, but that seems to be less of a problem now.