To prevent diapers leaking you should make the diaper snug but not too tight.
If you can slide 2 fingers under the waistband of your babies diaper then the fit of the diaper is just right.
Secure the diaper around the babies waist just snug enough to get 2 fingers under it and if it's too tight then loosen it up a bit.
If you notice any marks on your babies waist then you're putting the diaper on too tight.