You should move up diaper sizes when you notice that the diapers are starting to get too tight on your baby or toddler.
The weight on the diaper packages are just a guideline and not always accurate. Some babies might be bigger than others so different diaper sizes fit different weight of babies and toddlers.
You can also tell when to move up diaper sizes when you notice your baby or toddler is peeing through the diapers.
Bigger size diapers absorb more than the smaller size diapers.
Diaper should fit snug but not too tight. You should be able to get 2 fingers under the diaper waistband without a problem.