I didn't try the sized covers but I love the duo wraps. They fit my 6.10 peanut DD really well, and she still fit them close to a year and 20 lbs. My DS outgrew the size 1s at 2 months. If I had bought a bunch of xs covers for him he would have barely spent any time in them. The only time I would consider the xs was if I had a preemie... I've heard they fit pretty tiny babies.
Posting from my iPhone, hope it's legible
Mommy to my sweet little "Peanut" (4/22/10) and little man "Cashew" (9/28/11)