One size diapers are exactly as they sound. They are meant to fit a child from birth until about potty training (or 30-35 pounds).
This sytle diaper typically has a row of snaps on the front of the diaper for a small, medium, or large setting. Some One size diapers can be changed from small to large with a leg adjustment inside the diaper. As the baby gets bigger, the snap settings or leg adjustment allow the diaper to get bigger around the waist and/or leg area.