Dear Magnus,
Since the Tourbillon Trilogy was designed from the outset as a complete set of three tourbillon types, it was possible to determine the exact placement of the tourbillon and sub-dials fitting to the 3 different case shapes. Keeping this placement the same insures the visual harmony of the set as well.
Additionally, the idea was also to allow the tourbillon to viewed from front as well as from the back and since all axes are flying, the motion of the tourbillion is fun to watch. So it was logical to provide a clear and open area to accentuate this.
The diameter of 43mm and the height of about 16.1mm remains the same in all three. This height is similar to many chronographs. This low height was aided through the use of the flying tourbillon construction.
Regards
Thomas