I think Richard T has made a god point - the total population of Aberdeenshire is less than 250,000 - let's be generous and say the Aberdeen Apt catchment area is 400,000
This is less than a quarter of that of (say) Newcastle - and they struggle to expand as well. In fact it's only because of the distance to London and the high value oil related travel that ABZ does as well as it does . Ideas of major expansion are dreams -- if they're lucky they might eventually get an Emirates flight 2-3 times a week