Hasselblad X1D has a 51.0MP Medium format (44 x 33 mm ) sized CMOS sensor . On the other hand, Sony A6600 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz X processor.
Hasselblad X1D's sensor provides 27MP more than Sony A6600'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 A6600 sensor size comparison.
As seen above, Hasselblad X1D has a 4.0x Larger sensor area than Sony A6600. 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.
DxOMark Sensor Scores
Both Hasselblad X1D and Sony A6600 sensors have been tested by DxoMark. DxoMark scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Of the two cameras that we are comparing, X1D has scored 102, 20 points higher than A6600.