The story
From the notion that the diaper is intended to fit the baby from, or near, birth until they are toilet-trained, thus coming in only one size; compare one-size-fits-all.
Etymology from Wiktionary, CC BY-SA 4.0.
one-size diaper
Today it means: A type of cloth diaper which can be adjusted and lengthened to match the baby's growth, thus allowing the diaper to be worn from around birth (by a newborn) to potty-training age. Full definition →