Back in the early days, the first aircraft were some very old B737-300s, then EZY got their brand new 300s, to be followed by the 700 which I was closely involved in introducing. The philosophy from management then was that if the smaller aircraft was full then put on more flights. As the major airports we flew to became more slot constrained, bigger aircraft were needed to take the demand, thus the 320 was introduced. In fact I forecast the use of the 321 as well before I left and long before EZY got any.. We operated the ex-GB airways 320s for a while, but not their 321s. Now the final version of the family has been added and seems to be doing very well.