Both Panasonic ZS1 and Panasonic LX5 have 10.0 MP resolution sensors but Panasonic ZS1's sensor is 1/2.5-inch (5.744 x 4.308 mm ) and Panasonic LX5's sensor is 1/1.63-inch (8.07 x 5.56 mm ).
Since Panasonic LX5'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 ZS1.
Below you can see the ZS1 and LX5 sensor size comparison.
As seen above, Panasonic LX5 has a 1.8x Larger sensor area than Panasonic ZS1. 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.