Why do boys sweat more than girls during exercise?
Your assumption seems to be true that Males sweat more than Females, but, not necessarily because of muscle mass.  The jury is still out as to the reasons.  However, in a 2010 study, it was concluded that…


  … training improved the sweating response, and a sex difference was
  observed in the degree of improvement in the sweating response due to
  physical training. This sex difference became more pronounced with
  increasing exercise intensity. A sex difference was observed in the
  control of sweating rate to an increase in exercise intensity, i.e.
  the maximal activated sweat gland responses of untrained females
  required a higher body temperature or work intensity than the other
  groups.


So, there does seem to be something different in the way males and females sweat.  Recent research suggests


  that when we're under heat stress from high temperatures or an intense
  workout -- regardless of what shape we're in or how much body fat we
  have -- women consistently sweat less than men but tend to have a
  higher skin temperature.


(http://health.howstuffworks.com/wellness/men/sweating-odor/men-sweat-more-than-women.htm)

In a nutshell, the cause(s) for the differences in how men and women sweat is still unknown.  Some think it may have to do with body composition.  While others think more research is required to determine


  how efficiently we use the sweat we produce, but also to our capacity
  for making sweat, hormonal fluctuations, sex hormone differences and
  how our bodies regulate water as possible reasons why men sweat more
  than women.