Obviously it is school holidays again........
The reason is that the A319 is smaller and lighter than its larger stablemates, and therefore burns less fuel per hour. Likewise the A320 is about 12 tonnes lighter than the A321, and hence burns less fuel per hour than its larger sibling.
They all cruise at the same speed. Therefore, the less fuel you burn per hour, whilst traveling at the same speed, the longer you can stay aloft, and hence the further you can travel.
This is a simplification, but essentially how it works.