[RedDevil]

For the record, an excellent write-up about alternator efficiency and construction can be found here: http://www.delcoremy.com/documents/h...ite-paper.aspx

Typically, alternators have a voltage regulator that drives the secondary coils by PWM modulation; see page 13 and 14 in that link.
Instead of burning off excess voltage though a resistor, PWM switches the power on and off to match the current required with minimal loss.

The real reason why alternator deletes are efficient is that the whole chain from gasoline through engine efficiency, belt efficiency and alternator efficiency means that just 21% of the potential energy gets converted to electricity - at best. (page 3 and on)

But the alternator is not the worst part, averaging 55% efficiency on its own; nor would a smaller alternator help out much. The alternator should be dimensioned to be able to provide the maximal current the car can draw, but will be most efficient at half that current or less when operating in the 2000-3000 rpm range, where it would be most of the time.
A small alternator that can barely provide the 'normal' current draw would be on its toes and in fact be less efficient than a somewhat bigger one.
(a graph on alternator efficiency per rpm and load can be found on page 23).