I am currently helping to restore a Weber Duo-Art grand, and
I restored the stack a while ago.  It, too, had graduated pneumatics,
this time four different sizes, getting longer towards the bass end
(i.e., no relationship to the length of pushrod).

My view is that it relates to the weight of the hammers, which
increases considerably from treble to bass.  They do all have the
same span, since the piano action has the same movement on all notes.
The extra length increases the area over which atmospheric pressure
operates when the pneumatic closes, thus increasing the force applied.

John Farmer, UK