Most of the obvious visible changes were actually made last year with the FW45 Canada-upgrade:
Williams said the FW46 wouldn't necessarily look drastically different to its predecessor, and indeed it doesn't. On the surface, it's mostly evolution rather than revolution. Without more images from different angles, I'm not certain how much has actually changed in some areas like the sidepod. The front wing and floor edge are probably the most significant visible changes. The big changes are obviously under the skin, on the underside, and in how the car was developed.
Williams have finished the job started last year of flattening and broadening the nose. Instinctively, you would perhaps expect the evolution to be from bulky to sleek and not the other way around. But this is the direction we've seen several teams go.
It looks to me like it could just be down to the difference in focal length and perspective. These photographs can be deceptive, because rather than being shot from far away with long telephoto lenses as usual, they were shot relatively close-up with a wider-angle lens. That will make features in the background look smaller. The images we'll get in the coming days should clear things up.