No need to have studied electrical engineering, it is enough to have seen a 12 kW motor or alternator once to know that weight and volume are absolutely not comparable to those of a car alternator.
And with a little bit of mechanical engineering it is easy to calculate the force exerted on the belt by a 1100W alternator (+/-80A) at full load at 2000 rpm for example, it is +/- 17 lbs .
If you're interested I can tell you the details of the calculation, it's easy to understand by everyone, there are no complicated mathematical functions, only multiplications and divisions.