Nikon D810A has a 36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features EXPEED 4 processor. On the other hand, Hasselblad X1D has a 51.0MP Medium format (44 x 33 mm ) sized CMOS sensor . Hasselblad X1D's sensor provides 15MP more than Nikon D810A'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 D810A and X1D sensor size comparison.
As seen above, Hasselblad X1D has a 1.7x Larger sensor area than Nikon D810A. 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.