Torque is a trade with speed.
Smaller wheel diameter gives
more torque an the expense
of top speed.
Torque is force times radius
But you have two torques, one
For the motor, call it Tm,
And one for the tire, call it Tt
(hee hee, t-t)
Your torque is Tm/Tt.
The pi3 gives a higher Tm, so
you get more torque.
A smaller rim increases your torque at the
same power, but there will be danger in
running over speed.
IF I had money and a machinist I trust,
I would add a secondary chain driven
Motor. Similar to
http://www.ecospeed.com/But a lower power motor at the correct
voltage to switch in, and not in that location.
(it should freewheel the pedals so they don't go nuts.)