Hasselblad X1D has a 51.0MP Medium format (44 x 33 mm ) sized CMOS sensor . On the other hand, Sony A99 has a 24.0MP Full frame (35.8 x 23.8 mm ) sized CMOS sensor and features Bionz processor. Hasselblad X1D's sensor provides 27MP more than Sony A99'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 X1D and A99 sensor size comparison.
As seen above, Hasselblad X1D has a 1.7x Larger sensor area than Sony A99. 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.