On 14 Mar 2016, N_Cook wrote
(in article ):

That would be lead to a much closer pair of pulley diameters.
But then although you have a free range of pulley sizes, as cast and
machined, the sintered metal bearings come in only discrete sizes of * x
0.1mm , so you have to increase/decrease the pulley diameter ratio to
then be compatible to 2 available bearing diameters and hence spindle
diameters.
That is the reason i'd go for

I (op) have accepted the theory that difference is to avoid additive
resonance of the capstan shafts in bronze(?)--not ball--bearings. Difference
flywheel sizes plus different capstan shaftssizes (2mm and 2.1mm) results in
same tape speed (in opposite directions--this is an auto-reverse mechanism)
and different resonances.