NYC XYZ wrote:
Say, why is the woman always in the back on a tandem?

There are some tandem designs made for the smaller person to ride in
front. The Counterpoint tandem is one of example where the person in
back is in a normal upright bike configuration and does the steering
(plus pedaling) while the front rider is on a recumbent seat and just
pedals.

However, for conventional upright tandems it's generally a more
enjoyable experience if the heavier and stronger rider is the one in
front and takes care of the steering and braking. I have ridden our
tandem with heavier/stronger stokers on occasion, but it requires more
concentration to avoid having the motions of the stoker affect the
balance and steering.

Many regular upright tandems, including ours, are also designed with a
longer seat tube in front so it's a better fit if the taller rider sits
there.

..but
also, doesn't the stoker provide more power?

No, each rider puts out as much power as they wish although on most
tandems they must pedal at the same cadence. But the person in front
might be pushing really hard on the pedals while the one in back is
loafing or vice versa.