We show that the general limit of power scaling of thin-disk lasers comes not only from the overheating and amplified spontaneous emission (ASE) but also from the surface loss. Overheating or thermal fracture increases the transverse size at the scaling, whereas ASE limits the gain-size product. The gain coefficient should decrease at the scaling. However, the round-trip gain should remain larger than the background loss; hence, the thickness should increase at the scaling. The limit of the output power per single active element occurs when the medium becomes too thick and cannot work efficiently without overheating. The maximum output power scales inversely with the cube of the surface loss coefficient. In the quasi-continuous regime, the average power scales inversely to the product of the duration of pulses to the repetition rate.