DS2, who was nearly 10lbs at birth, outgrew the smalls around 7/8 weeks. He is 10 months old now and still in mediums. He is about 18lbs, BTW.
DS1, who is about 30lbs, could still wear mediums. He uses the potty now, but I had one on him last week for a picture.
with DS2, we never really had any period of overlap. We made due with the smalls until the legs fit on the mediums. By then the legs on the smalls were way too tight.