Both Canon G15 and Olympus TG-6 have 12.0 MP resolution sensors but Canon G15's sensor is 1/1.7-inch (7.44 x 5.58 mm ) and Olympus TG-6's sensor is (1/2. ).
Since Canon G15'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 Olympus TG-6.
Below you can see the G15 and TG-6 sensor size comparison.
As seen above, Canon G15 has a infx Larger sensor area than Olympus TG-6. 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.