My dd outgrew the mediums for a while when she was at that roly-poly chubbo stage. She only weighed probably 20lbs, but most of it was in her mid-section, lol. Once she started crawling and walking, she slimmed down and though she weighs more like 25-26lbs now she fits the mediums again very nicely. So, I think it probably depends on the shape of the baby. My dd has actually been able to fit into med or lg Bummis SWWs for a long time now, but I like the trimmer fit of the mediums better. Hope that helps!