how old is your dd? If you don't expect much weight gain before she potty learns I would go with medium they fit my skinny girl great until about 25lbs. My dd chunked up a bit though after my ds was born all that milk I guess lol and now the large fits her best with lots of room to grow.
My son is almost 26 lb (at 26 months even), and the medium bumpy fits him well. His waist is 16" and his thighs are between 9.5 - 10", and the thighs are coming close to the outer limit of the cover (though the leg openings stretch nicely). So I think the medium bumpy might be a bit tight in the legs for her, and i'd go with the large-- which will probably be pretty big, but the velcro panel in front allows for a good fit even when the cover is pretty big around the waist.