Both Canon 6D MII and Pentax K-3 III have 26.0 MP resolution sensors but Canon 6D MII's sensor is Full frame (35.9 x 24 mm ) and Pentax K-3 III's sensor is APS-C (23 x 15.5 mm ).
Since Canon 6D MII'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 Pentax K-3 III.
One other difference between these two cameras that is worth mentioning is that Pentax K-3 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 6D MII and K-3 III sensor size comparison.
As seen above, Canon 6D MII has a 2.4x Larger sensor area than Pentax K-3 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.