Sigma SD15 has a
5.0MP APS-C (20.7 x 13.8 mm ) sized CMOS (Foveon X3) sensor and features True II processor. On the other hand, Nikon D7100 has a
24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor .
Nikon D7100's sensor provides 19MP more than Sigma SD15'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.
Another similarity between these two cameras is that both Nikon D7100 and Sigma SD15 sensors lack anti-alias (Low-Pass) filters. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the SD15 and D7100 sensor size comparison.
As seen above, Nikon D7100 has a 1.3x Larger sensor area than Sigma SD15. 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.