One possible reason is to reduce the roll sensitivity in icing, which affects only the A321. This was discovered after multiple control issues and further investigation by Airbus:
Main findings were:
- Similar ice accretion on both A/C.
- Increased spoilers efficiency on both A/C.
- Increased roll sensitivity only on A321.
- Analytic roll inputs showed unusual A321 roll response of the iced A/C (overshoot, degraded turn coordination…). (A320/319 are OK on this
- This degradation was mainly observed in Conf Full, to a much lower extent in Conf 3.
Source: Airbus via smartcockpit.com
I skimmed the presentation but there doesn't seem to be a clearly stated underlying reason, though the 321's wing is slightly bigger.
If this is not it, then at least this shows a difference in roll performance between the 320 and 321. After all the 321 is bigger with higher MTOW, so the lift is also bigger when heavier, and the wing is only slightly bigger (i.e. higher wing loading).
Another possibility is the longer fuselage, which gives the rudder a bigger moment arm. Since spoilers in a bank counter the adverse yaw, having a bigger rudder moment arm would call for less spoilers.