Just like most airframes, they always get heavier as new models are introduced, its rarely the other way. New features like soundproofing, longer props, redesigned cowlings, bigger engines, bigger brakes and tires, more seats, thicker windows, more avionics, AP systems, a/c systems, larger fuel tanks, the list goes on and on and on....
To a certain extent, gross weight can be manipulated by a manufacturer. The Air Force O2 was essentially a 337, but the military version had a 1000# higher GW at 5400 vs. 4400. Same engines. I wonder why?
