Hasselblad X1D has a 51.0MP Medium format (44 x 33 mm ) sized CMOS sensor . On the other hand, Pentax K-1 has a 36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor .
Hasselblad X1D's sensor provides 15MP more than Pentax K-1'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 Pentax K-1'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 moire occurring in certain scenes.
Below you can see the X1D and K-1 sensor size comparison.
As seen above, Hasselblad X1D has a 1.7x Larger sensor area than Pentax K-1. 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 Pentax K-1 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, 6 points higher than K-1.