In 2014 170k movements carried just short of 22 million passengers - almost 129 per movement. In 1990 the average was around 86 per movement. So if that 1990 average had been maintained with today's passenger levels we would be looking at 50% more movements in a year - that would stretch the available apron space!
What is perhaps more surprising is that in summer, when there are much more aircraft movements (with less a/c just parked up) and more passengers, with the extra revenue they both produce for the airport, it is the increased demand for car parking that gets priority.
Perhaps not that surprising as the total stand occupancy time in July will be less than in January. Jet2 are perhaps the best example - out of a based fleet of say 15, on some winter days 10 may not fly at all, whereas in July that would be 1-2 max. So one stand that contains a single aircraft in January could contain 12 or more in July, assuming they only stay in for 1-2 hours, (and usually less when on a turnround). In winter, the based shorthaul fleet almost all night stops; in the summer a fair percentage will be away doing overnight trips, and if they don't return until after 6, a number of the nightstoppers will have departed, freeing up stands.
I realise that is oversimplifying things a bit, but I think the point stands (if you pardon the pun!).