The depth of the FT26 and FT36 cannot be related because it would make the FT26 and 36 huge monsters. Frankly, if it makes sense that the FT16 MIGHT have been named for its depth as it is close to 16mm (0.640" = 16.256 mmm), it certainly cannot apply to the other motors, and not even to the "other" Mabuchi can motor, the FT13UO.
Hence, the mystery remains.
Ron can elaborate on the advent of the Parma "16D", when and why it was born.
Was the FT13UO smaller than a current FK motor? A current FK motor is a little over 15 MM in height.
The Parma 16D... at some point in the '70s Parma and Champion started using the Johnson 222 16D motor. When Johnson stopped production of that model/motor in 1982-83 that left Parma scrambling to find a new source for 16D motors. In 1983-84 company was found in Hong Kong to produce a replacement. The early versions of these motors were not too great and had a lot of problems and over the years to follow many changes were made to produce a decent motor at low cost.
I can, at some point, post pics of the various "editions" of the evolution of the Parma 16D motor along with other tidbits and specs.