The internal diameter.
Apologies for all the post but hopefully this will help me explain the problem better.
The diameter of the mm bottle is perfect. Without the top cap I can fit the bottle in the pico and attatch the door. However the mouth of the mm bottle is too wide so the squonk does not work.
If I assemble the bottle it becomes too tall and the washer at the base of the squonk mechanism is too wide to fit through the top of the mm bottle.
The only way I can think to get the bottle to work properly is to remove the plastic washer at the top of the pico squonk mechanism as well as the flat metal washer at the base of the pico squonk mechanism.