It is correct that CPH actually is slightly smaller than ARN today.
What happened was that first the great belt bridge was built, then the train station at the CPH terminal 3, and finally the bridge to southern Sweden - a lot of "local" CPH pax are are from southern Sweden.
Domestic traffic in Denmark has almost collapsed - surface transport is often much faster and more convenient. Several routes have closed down, and most of the rest have changed from loads of MD-80 and 737-300 to ATR-42, F-50 and Dash-8.
Sweden is ten times larger and therefore has a natural and ever growing domestic traffic.
A very typical passenger at ARN is often in transit between a domostic feeder route and an international route. He counts as two passengers, but doesn't even see an ARN check-in counter.
On the other hand a typical CPH passenger mostly has CPH as his destination, at least as air transport passenger. He actually checks in there, but counts only as one passenger.
Therefore CPH may look larger, but it isn't.
In addition, in older days practically all Danish international passengers passed CPH. Today half of the population, who lives in western part of the country - Jutland - more and more use feeder routes from Billund, Aarhus, etc. to other hubs, Amsterdan, London, Frankfurt etc. and fly out in the world on BA, LH or KLM instead of SAS.
In the good old days CPH was a good transit hub with not very frequent missed connections caused by congestions. Today it is no better than the other large hubs in western Europe.
Add to that, that at all Jutland airports you just park your car and walk two minutes to the terminal. Parking a car at CPH at rushhour time is a mental - and economic - nightmare.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs