Toni-A, both companies sold more than 100 million lenses. Each of them. This makes decisions more difficult. Also, the Nikon mount doesn't allow lenses faster than f/1.4 if the electrical contacts have to remain in place. Nikon would be stupid to stick with this mount and sell empty space (not more needed for a mirror). But if a new lens mount is inevitable, each other paradigm can also be questioned as it's a decision for the future.
And becoming compact but also FF or bigger doesn't necessarily belong together. It still has to remain operatable. Fiddly buttons are no goal and lengthy menus are not professional either. So, a certain surface is needed. I see the Sigma SD quattro as well designed although not compact- but the buttons are well placed. I understand that for Sigma it would be hard to leave the people which invested in the exotic Sigma mount buy another set of slightly more compact lenses. Slightly, because they already try to increase the sensor size (APS-H). And if they try to approach FF size, it would b better not to start with smaller lenses in even smaller quantities, but stick to Sigma mount and allow other Sigma lens owners a mount change. That is still cheaper than selling old and buying new.