As has been said, red wine glasses are generally wider because
increased exposure to air helps the deeper and generally more
complex flavors of red wine develop fully. Larger surface area
means higher air exposure, and the wider bowl of red wine
glasses allows for more wine to be exposed at any given time. In
general, white wines do not require as much oxidation for the
flavors to expand, thus partly explaining the narrower shape.
However, there are other reasons. Heat transfer is another big
reason for the more narrow shape of most white wine glasses.
Larger surface area means higher oxidation, but it also means
that more heat will be transferred into the wine from the
surrounding air. The larger the surface area of a
cooling/warming object, the higher the rate of heat transfer, so
having a narrower opening decreases the exposed surface area,
ultimately keeping the wine chilled longer. One of the biggest
reasons, however, is flavor distribution. Different kinds of
glasses are made to deliver different sorts