Proraps ran true to size for us. The Bummis fit well, but I don't remember how accurate the sizes were. Litewraps were HUGE--as in absurdly big in the seat. Sprogly is still wearing them at well over 40lbs even though they say up to 30lbs. His height/weight are proportionate--same percentile for both.
Sprogly's nearly done with diapers, and even though the litewraps are the only covers I have that still fit him, and I refuse to buy any more, I don't like them and don't recommend them. They don't come as clean, and the snaps have all come off. I liked Proraps the best.