I'm not sure what E series switches you are talking about vs. what C series switches you have measured. None of my E series switches (3 that I pull up from various E series lights at hand) are even close in OD to the size of any of my C series switches (8 C/Z/P tubes with switches at hand). The difference is clearly visible to the naked eye and I have checked that there is a measureable difference using a pair of locking pliers (man I wish that I could find my calipers in the hole that is my garage!). The C series switches are flush in diameter to Leef's 18650 tubes. The E series switches on the TB tubes that I have are far smaller in OD than the tube itself. If I place a C series switch in place of the E series switch on my 2 TB tubes, the C series switch looks way more proportional.
While a C switch will still not allow the new 'C' cell to fit through the tail, it wil be much more proportional than any except for an extreme custom E series. Not that an extreme custom E series is bad, its just that it adds to the cost without any more functionality. Leef switches are C sized and use E series internals, so the two level issue isn't true for all C series switches. There are quite a few variations that Leef has made available at the Hound...
Anyway, just a thought.