Slim - many thanks for the list. I wasn't sure on one thing though - do the number refer purely to what lies in the official municipal boundary (probably the case, given that Paris looks comparable to Bucharest rather than London), or are they a measure of catchment area, showing number of people within perhaps 75 mins travelling time ?
Further, do you have figures combining both catchment area AND a proxy for disposable income (GDP per capita might be a start) ? Again, Paris should trounce Bucharest on this measure.
Other factors I was trying to think about are:
1) Not going head-to-head with another LCC when an airport probably can't support 2 substantial LCC operations - e.g. Budapest and Warsaw with large Wizz operations
2) Avoiding cities where there is only one airport for miles around that also happens to be a major hub for another airline (e.g. Munich). I consider Madrid to be an exception because of the newly released space in the old terminal
3) Looking for airports where the dominant airline is in a weakened state - easier to grab market share