Both Sony A7 and Pentax K-70 have 24.0 MP resolution sensors but Sony A7's sensor is Full frame (35.8 x 23.9 mm ) and Pentax K-70's sensor is APS-C (23.5 x 15.6 mm ).
Since Sony A7'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 K-70.
One other difference between these two cameras that is worth mentioning is that Pentax K-70'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 A7 and K-70 sensor size comparison.
As seen above, Sony A7 has a 2.3x Larger sensor area than Pentax K-70. 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.