Both Fujifilm X10 and Pentax MX-1 have 12.0 MP resolution sensors but Fujifilm X10's sensor is 2/3" (8.8 x 6.6 mm ) and Pentax MX-1's sensor is 1/1.7" (7.44 x 5.58 mm ).
Since Fujifilm X10'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 MX-1.
Below you can see the X10 and MX-1 sensor size comparison.
As seen above, Fujifilm X10 has a 1.4x Larger sensor area than Pentax MX-1. 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.
DxOMark Sensor Scores
Both Fujifilm X10 and Pentax MX-1 sensors have been tested by DxoMark. DxoMark scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Of the two cameras that we are comparing, X10 has scored 50, 1 points higher than MX-1.