Some possible explanations
1. Longer fuselage = more longitudinal stability. Both in yaw and pitch.
2. Longer distance from the stabilizers to wing - more stability. Especially when extending flap
adding alot of pitching moment.
3. The A320 should have a higher Vref than the A319 for the same weight to avoid tailstrike (at least on the 737NG's thats the case). So you will always fly relativly slower with the A319.
Cheers,
M