Well certainly the cartridges have a minimum distance between primer centers. Since the web between the chambers must survive a double discharge, whatever that thickness is determines how wide the barrel assembly can be.
The standing breech must be heavy enough to defend against the cartridges without tearing loose of the action flats. It needn't be wider than that minimum barrel set width. That's about as narrow as a barrel set can be.
You can make everything behind that as miniature as the operator can manage, provided you can set off the primers, and reach the trigger.
At a certain point it's kind of like a phone. It has to reach from the ear to the mouth.
