Both Fujifilm XQ2 and Panasonic ZS50 have 12.0 MP resolution sensors but Fujifilm XQ2's sensor is 2/3" (8.8 x 6.6 mm ) and Panasonic ZS50's sensor is 1/2.3 (6.17 x 4.55 mm ).
Since Fujifilm XQ2'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 Panasonic ZS50.
Below you can see the XQ2 and ZS50 sensor size comparison.
As seen above, Fujifilm XQ2 has a 2.1x Larger sensor area than Panasonic ZS50. 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.