Randy, it has to do with the diameter of the tube.
For each format you have to use the "right" diameter, so that the film curls inside the tube in such a way as to not being pushed completely flat against the wall of the tube.
The 18cm dimension of the films I'm using has the right amount of tension/freedom inside a tube with a 3" diameter.
This allows for the chemistry to reach both sides of the film.
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