Bigger sensors, more light
The full frame medium format sensors found in the current range of IQ Digital Backs are 2.5x larger than that of full frame DSLR cameras, and 1.5x larger than cropped sensor mirrorless medium format.
The use of a larger sensor means more light, and therefore more information, is available when converting captured light into a digital file. The availability of more data allows us to honor the quality of that data, translating to a more efficient use of information with less interpolation.