Sigma fp L has a
61.0MP Full frame (36 x 24 mm ) sized BSI-CMOS sensor . On the other hand, Nikon D800E has a
36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed 3 processor.
Sigma fp L's sensor provides 25MP more than Nikon D800E'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.
One other difference between these two cameras that is worth mentioning is that Nikon D800E's sensor doesn't have an 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 moiré occurring in certain scenes.
Below you can see the fp L and D800E sensor size comparison.
Sigma fp L and Nikon D800E 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.