Heat tries to distribute itself as equally as possible. 1 Therefore, smaller heaters concentrate the heat and larger heaters spread it out resulting in lower surface temperatures. Wein's Law of Displacement states that the wavelength is inversely proportional to temperature of the source. 2
1. Astronomical Institute / Utrecht University . 11-14-04. http://www.astro.uu.nl/ strous/AA/en/antwoorden/temperatuur.html
2. Nobel Lectures. Physics 1901-1921. Elseyler Publishing Company, Amsterdam. 1967.
Note a round sauna will give the best rebound of sauna waves back to the bather becuase of the circle nature.