You may get more complete answers from others, but keep in mind that all digital 645 cameras are a crop from 645 film - especially the Pentax and the new models from Phase One and Hasselblad which use the same senor as the 645Z. Phase One, for example, has a number of sensors that are less of a crop - including one or two sensors that are a fairly minimal crop.
Here is a size comparison, though you can find more comprehensive information comparing different brands on Wikipedia:
http://www.phaseone.com/Camera-Systems/IQ2-Series/IQ2-Specifications.aspx
The IQ250 uses the same sensor as the Pentax 645Z. The IQ250 certainly has its advantages, but it costs a lot more than the Pentax body - or for that matter a complete Pentax system! Note that while Phase One calls the "Lens Factor" 1.0 on the larger sensors, they still aren't quite as large as the frame of 60x45mm on film (let alone the larger frame that same film gives in a 67 camera).