The Shimano SM-BB9000 bottom bracket features a 25 gram weight reduction over the 7900 plus a new black-anodized finish. To achieve this considerable weight loss the external cup size is noticeably smaller thanks to smaller bearings and a reduced external diameter.
Other than the smaller cup and bearing size, the system is still Shimano's well proven Hollow Tech II design with a 24mm internal diameter and the benefits of a wider bearing platform, easy installation and serviceability. Improvements don't stop at weight reduction. New bearings and seals allow a claimed 50% reduction in friction so your cranks will spin more freely plus there is no pressing need to jump to ceramics.
As side note - your standard outboard bottom bracket tool won't fit directly due to the smaller external diameter. But the good news is Shimano includes an adapter that nicely works with all the standard outboard bottom bracket tools. Select BSC (also known as English) or Italian threading.