The cab is bigger than the old one, actually -- it's wider for sure.
The cabs, as mentioned, are sealed and pressurized; the hydrualic system got a similar bump in performance. The breakout force on the large-frame models is now over 7,000 pounds, a large improvement over the 5,500 lbs of the B-series vertical lift machines and the 4,000-ish pounds of the radial machines.
The torque tube (cross member) of the loader arms is a long, long story... I had the same question, but I'm satisfied that Cat did the right thing. There is a balance and a close relationship between a narrow cab (Case), bent loader arms (Bobcat, Deere, NH, ...), the loader arm stops, and the amount of protection to the bucket cylinders. The prototype machines -- the front end especially -- looked very different, but the current design reflects a lot of engineering iteration and reiteration to solve problems from the prototype builds.
The 246C is a heavier machine than the 246B.