Both Panasonic GH5S and Panasonic LX5 have 10.0 MP resolution sensors but Panasonic GH5S's sensor is Four Thirds (17.3 x 13 mm ) and Panasonic LX5's sensor is 1/1.63-inch (8.07 x 5.56 mm ).
Since Panasonic GH5S'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 LX5.
Another difference between these two cameras is that Panasonic GH5S's sensor lacks 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 GH5S and LX5 sensor size comparison.
As seen above, Panasonic GH5S has a 5.0x Larger sensor area than Panasonic LX5. 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.