Nikon D80 has a 10.0MP APS-C (23.6 x 15.8 mm ) sized CCD sensor . On the other hand, Nikon D780 has a 25.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor and features Expeed 6 processor.
Nikon D780's sensor provides 15MP more than Nikon D80'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 D80 and D780 sensor size comparison.
As seen above, Nikon D780 has a 2.3x Larger sensor area than Nikon D80. 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.