Archies Jandals are designed with a ‘snug / tight’ fit in mind. They should ‘fit like a glove’ with very little space in front of your toes and behind your heels. This will ensure that the arch support is in the correct position on the foot.
It’s not a concern if a small amount of the back edge of the heel hangs over the edge of the jandals (as per the below image).
If your heel is completely over the edge of the heel cup and squashing down on the rim of the jandal, then the size is too small for you. However, some heel overhang is ok.
The most important thing is that the jandals are comfortable and the arch support is in the correct position.
Sometimes it can seem that the fit is too small when it is actually perfect. In this case, it’s most likely that the tight strap and toe pole that’s stopping you from getting your foot all the way into the jandal. To fix this, simply stretch the strap with your hands.