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 20Da has a
8.0MP APS-C (22.5 x 15 mm ) sized CMOS sensor .
Canon 60Da's sensor provides 10MP more than Canon 20Da'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 20Da sensor size comparison.
Canon 60Da and Canon 20Da have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.