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.