Diapers should fit around baby's belly and legs snugly. Don't worry- you won't hurt them! You should be able to fit a finger in, but in order to do so, you should have to indent baby's fat. It should not be easy to get a finger in! If a finger can get in, pee can get out. If it's on snugly, try changing your baby more often. Babies (and big kids and adults) should be changed at least every two hours, in addition to when they soil themselves. Bacteria starts to grow at about that two hour point, so change away. Overnights should be ok, just make sure to wipe baby well in the morning. Also, make sure that none of the insert is sticking out the back- if it is, tuck it in, and pull the cover over it.