A diaper should fit on a baby so as not to be too tight, too snug, and the diaper should cover the babies bottom and come up to or slightly above the babies belly button.
A properly fitting diaper should fit snugly around the babies waist and legs and bottom but it should not be too tight.
Too small of diapers could be too tight and cause rashes, marks and maybe constipation.
Diapers that are too small will also leak as the diaper does not have the capacity of a larger diaper.
When put a diaper on your baby or toddler you should be able to get 2 fingers under the diaper waistband and the diaper should come up to or above the belly button.
Also you should run your fingers around the leg gathers of the diapers to help prevent leaks.
Tight diapers on your baby or toddler should not cause constipation.
However when the diaper is too tight then it can cause the baby or toddler to become uncomfortable cause rashes and red marks around the child's waist.
Tight diapers can also make it hard for the baby or toddler to move around especially if they are walking it can prevent them from sitting down or bending over easily.
So the diaper should not be too tight anyway and if the diaper fits tight then you may need to change diaper sizes and go up a size larger in diapers.
You should be able to fit 2 fingers easily in the diaper waistband between the diaper and the child's waist.
Your baby or toddler may not be getting enough fluids which can lead to constipation.
Most cases of baby constipation or toddler constipation go away on their own within a week or so but if not you should take the baby or toddler the doctor.