Lets say you use a 6mm bit and the material is 10mm thick, then cutting along, or even to the outside of the line, means the slot is still too thin for the pieces to slot together.
This is why I originally said that the first method is to use the same sized tool as the thickness of the material.
If you changed the drawing, as I suggested in the second method, this means you can use any sized tool on any sized material, so long as the tool was thinner than the material. You then use the cut outside the line option.
I Hope the drawing explains this.
Qualify: I would only use the edit option if you knew you wanted to make a few of the same design, and you wanted to use a smaller tool to reduce cutting and mess and strain on the machine.
A smaller tool is better for cutting than a bigger tool.