Here class D losses are I^2.R in the MOSFETs, and I^2 is proportional to power. A
good class D amp with discrete MOSFETs will be ~ 90% efficiency at full power.
The dissipation is likely shared between I^2.R, switching and inductor losses.
No single chip calss D amp can have low resistance MOSFETs integrated as these are
vertical current flow devices with the substrate being one terminal.