EZY fleet and route strategy is well articulated in their annual report, in a nutshell for fleets, its larger aircraft and lower seat costs, dumping the 319 in favour of 186 seat 320 and 320 neo:
"Efficient fleet management. We operate an exclusively Airbus A320 family fleet. This delivers operational flexibility as well as efficiencies in engineering and maintenance, crew, ownership and fuel. As the second largest operator of Airbus A320 family aircraft in the world we also benefit from significant economies of scale on acquisition. Between 2016 and 2021 we will derive a major benefit from up-gauging our fleet, from a majority 156-seat A319 composition to a fleet that is over 70% 186-seat A320s. The 186-seat A320neo aircraft are expected to have a 13% to 14% cost per seat benefit(6) compared to the 156-seat A319s."