A good way to tell when a diaper is too small and is getting tight around the babies or toddlers waist and legs is if you have to stretch the tabs to fit.
Or if you notice red marks around the waist or legs of the baby or toddlers body.
The diaper fitting too tight can cause red marks around the waist and legs.
Too small of a diaper can also cause leakage of urine and poop because it's not absorbent enough and is sitting too tight around the body.
Also if the diaper is looking kind of like a bikini around the back of the baby or toddlers body then the diaper is too small.
You should be able to fit 2 fingers easily in between the waistband of the diaper and if not then the diaper is too small and you need to go up a size.
When a diaper is too big on your baby or toddler the diaper tapes or diaper tabs will easily overlap each other and the leg gathers will sag and have a gap in them.
When the diaper is too big the diaper can leak and sag on the baby or toddler and then you'll have more poop leakage or urine leakage.
A diaper should fit snugly around the baby or toddler waist and there should not be any gap in the leg gathers or at least a very little gap.
The leg gathers help keep the urine or pee in the diaper as well as poop in the diaper especially when the baby or toddler is in a standing or sitting position.
When the diaper fits snugly you should be able to pull at the leg gathers and they should snap back and close closely around the legs.
The tabs of the diaper should fit easily but not too tightly around the waist.
You should be able to fit at least a few fingers into the waistband easily when the diaper is tight enough.