Some people tend to sweat easily, others stay almost dry. What determines how much a person sweats?
Humans are so-called warm-blooded organisms that are able to maintain a certain core temperature independently from environmental conditions. If the core temperature surpasses a certain limit due to hot weather or excessive heat generation following exercise, the body needs to regulate its temperature. Sweat drops on your skin absorb energy from the body (heat) when evaporating, hence cooling the body.
A recent study concluded that the sweat rate is related to core temperature change and heat produced rather than ‘fitness factors’ like body fat percentage or VO2 max. The latter is the amount of oxygen the body can use per kilogram and is often used to measure fitness levels. The study showed that the overall sweat rate is significantly correlated with the amount of heat produced per kilogram of body mass. Adding the body fat percentage and VO2 max did not increase the correlation substantially.
If two people exercise for the same amount of time, the fitter person would probably sweat more because more work is done in the same time and more heat gets generated.
The reason why overweight people tend to sweat more is that they have to haul more weight around and therefore produce more heat. The common myth that fat insulates the body and keeps heat inside is also false, since the amount of sweating did not relate to body fat percentage.
Science tries to measure body processes as accurately as possible but there is still a lot of variation in thermoregulatory responses. These can give you an overall idea but it is still up to you to trust your experience on how your own body copes with exercise and heat.