Both Canon 70D and Canon 6D have 20.0 MP resolution sensors but Canon 70D's sensor is APS-C (22.5 x 15 mm ) and Canon 6D's sensor is Full frame (36 x 24 mm ).
Since Canon 6D'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 Canon 70D.
Below you can see the 70D and 6D sensor size comparison.
As seen above, Canon 6D has a 2.6x Larger sensor area than Canon 70D. 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.