One-sized inserts create less complication and confusion, and result in more financial savings for parents. Babies grow so quickly that having an insert that is able to provide what you need from around 4kg through to toilet training years is important.
The one-sized insert will naturally appear bulkier on a newborn than a toddler, however they will grow into the bulk rapidly, and it means parents do not need to monitor the sizing differences and worry about purchasing more inserts as their baby grows.