Heat resistance is approximately a given (surface area), so wattage depends on the permissible temperature difference between the two items (die and heatspreade

Heat resistance is approximately a given (surface area), so wattage depends on the permissible temperature difference between the two items (die and heatspreader). If you run hotter, you can dump more power into it. Temperature is ultimately limited by sped up diffusion of doping and such, temperature limits in surrounding materials (eg epoxy-based chip carriers or epoxy-based case material), and reduced performance due to increasing leakage currents.