Available "medium formats" are 49x49, 54x40, 33x44, 45x30, 48x32
The format diagonals for 35mm, 44x33, 645, 6x6 and 6x7 are 43mm, 55mm, 72mm, 79mm and 88mm respectively. There has never been precise definition of medium format, but in film days it was easy to say where medium format began because there was, for all practical purposes, nothing between 135 and 120 (the exception was 127). Now there is quite a lot between 35mm and 645, and, of course, for your own purposes you can place the dividing line wherever you like - although, for the record, the biggest gap is between 44x33 and 645.
The point, however, is that for marketing purposes the dividing line can't be placed arbitrarily. The term "medium format" is being inappropriately used by Fuji (and Hasselblad) to borrow the mystique and prestige of film medium format for a sensor that is only a shade larger than 35mm.