The AP caliber from the 80s was slightly slimmer than the 1120QP (3.95MM vs 4.05MM) but the key benefit of the VC iteration is the ability to visually know in that year you are adjusting the calendar whereas AP did not display the year on the dial at all, and so any adjustment had to be done in good faith that the year is correct or by a watchmaker who takes the watch apart. The 1120QP is thinner than most alternative
It was a balance functionality AND thinness in a full rotor movement that won out here.