The overhead lockers on the A320 are bigger than the B737 where Ryanair normally have to put cabin baggage into the hold once the they bulk out at around 90 pax.
A320 typically only requires a few bags to go underneath and this normally only occurs with a full cabin or on certain routes which are baggage heavy.
Obviously a money making idea as they have little need to restrict hand luggage, unlike Ryanair who need
to reduce cabin baggage to avoid having to put it in the hold with the increased handling and turn around time.