Why do you think that AMS is so large, when the Netherlands is so small? Every time i pass through there, the majority appear to be Brits interlining through from Bristol, Glasgow and Newcastle, because it is easier to go to AMS than trying to crawl your way down to LHR. And that is all revenue that is lost to UK carriers, lost to the City of London, and lost to UK PLC.
It's also a way of avoiding the punitive UK Airline Passenger Duty on the long haul part of the journey.