Nikon D7100 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor . On the other hand, Canon 6D has a 20.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Digic 5+ processor.
Another difference between these two cameras is that Nikon D7100's sensor lacks 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 moire occurring in certain scenes.
Below you can see the D7100 and 6D sensor size comparison.
As seen above, Canon 6D has a 2.4x Larger sensor area than Nikon D7100. 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.