We report the behavior of two Yb(3+) doped ceramics (i.e. 10% at. and 20% at.) under quasi-continuous wave laser pumping. Two different behaviors are found depending on the density of Yb(3+) in the excited level. Experimental results show that at low population inversion density, the maximum output power and the efficiency are almost independent on the doping concentration. In particular, an output power as high as 8.9 W with a corresponding slope efficiency of 52% with respect to the injected pump power was reached with the 20% at. sample. Conversely, at high population inversion densities, the 20% doped sample shows a sudden decrease of the laser output for increasing pump power, due to the onset of a nonlinear loss mechanism. Finally, we report a comparison of the experimental results with numerical simulations for the evaluation of the inversion density and of the temperature distribution.