Remember that the A318 is not just an "even smaller A319".
The 319 sacrifices capacity to gain longer range than the 320.
But the 318 is the low capacity short range version of the 320 family, really a regional plane. It benefits from the small, cheap, economic and leightweight P&W 6000 engine. But derated CFM56 engines are an option for airlines wishing that extra maintenance commonality.
In other words, the 318 is much more a head on competitor to B717 and ERJ-190 than a shrunk 319.
Rgds, Preben Norholm
Always keep your number of landings equal to your number of take-offs