The bed moves along one axis, the head moves along the other ... that gives you the x,y axis movements. Additionally, the lens zooms. The reason your resolution is limited on larger sizes of film is that, even with the combined movement and zooming, after a certain point, you become limited by the ccd center sweet spot.
As an addendum there are a couple of older high end flatbeds that stitched in one direction but they were never in the leader class.
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