Both Leica Q-P and Ricoh GR III have 24.0 MP resolution sensors but Leica Q-P's sensor is Full frame (36 x 24 mm ) and Ricoh GR III's sensor is APS-C (23.5 x 15.6 mm ).
Since Leica Q-P's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Ricoh GR III.
One other difference between these two cameras that is worth mentioning is that Ricoh GR III's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the Q-P and GR III sensor size comparison.
As seen above, Leica Q-P has a 2.4x Larger sensor area than Ricoh GR III. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.