I was wondering the same thing. It isn't movement volume:
1100: 2779 ci
770: 1391 ci
440: 625 ci
It isn't table area:
1100: 323 in^2
770: 208 in^2
440: 113 in^2
The closest thing seems to be motor Watts:
1100: 1.5 hp = 1119 W
770: 1.0 hp = 746 W
440: 2/3 hp = 497 W
Or maybe weight (without stand)
1100: 1130#
770: 662#
440: 450#