When a diaper is too big it will fit loose and not as snugly as it should.
The diaper may also leak out the leg gathers and the diaper may slide down and sag on your baby when it's too big.
Also if you notice chafing around the kids legs when they are in diapers then the diaper could be too big and is rubbing against the kids legs.
If you have to tape the diaper on with the Velcro and they both overlap then the diaper is also too big and you should go down a diaper size.
Some brands of diapers fit differently even when they are the same size.
A diaper that fits right should not leak out the leg gathers, should fit snugly but not too tight and should not droop down on the baby.
The Velcro of the diapers tabs should not overlap each other but they can fit closely to each other.