|For what it is worth, My daughter weighs in at around 22 pounds at 20 months old. she has been wearing small sandy's and large Sandy's for at least 4 months now. We put m, ml and large covers on her still, and are able to fit not only either sizes of Sandy's but also the os with no problem. I really think it must depend on the shape/build of their bodies. My husband even manages to get a large Sandy PLUS a fat doubler into a medium for overnight with never a leak. and, actually, I do like the fit of the medium covers over the diaper, because it seems to make it a little trimmer. Best of luck to you!