I am not sure if I understood this correctly: "the volume is small but the heat source does not change". But as far I understand, the .mat file that is imported as the heat source will contain the power as W/m^3 which is the absorbed power from the FDTD( see the underlined text in the screenshot of analysis group from the FDTD):
As the input power is given in terms of power density, the amount of heat source will be scaled according to the dimension of the heat source area. So heat source should change. Please clarify and thanks for your help.