Both Canon ELPH 150 IS and Olympus E-M1 II have 20.0 MP resolution sensors but Canon ELPH 150 IS's sensor is 1/2.3' (6.17 x 4.55 mm ) and Olympus E-M1 II's sensor is Four Thirds (17.4 x 13 mm ).
Since Olympus E-M1 II'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 Canon ELPH 150 IS.
One other difference between these two cameras that is worth mentioning is that Olympus E-M1 II'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 moiré occurring in certain scenes.
Below you can see the ELPH 150 IS and E-M1 II sensor size comparison.
As seen above, Olympus E-M1 II has a 8.1x Larger sensor area than Canon ELPH 150 IS. 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.