Canon 60Da has a 18.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 4 processor. On the other hand, Canon 6D MII has a 26.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features DIGIC 7 processor.
Canon 6D MII's sensor provides 8MP more than Canon 60Da's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the 60Da and 6D MII sensor size comparison.
As seen above, Canon 6D MII has a 2.6x Larger sensor area than Canon 60Da. 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.