In reply to nw:
It's what my lecturers taught me when I was at Uni. As you get fitter you get more efficient, that's more or less the definition of fitness; your ability to do a task. If the task involves regulating body heat, then turning the sweat on and off quickly is more efficient.
There is always some level of individual variation and some people do just sweat more than others, but the amount will change with their fitness level.