From what I can surmise from browsing current SAS short to medium haul schedules, aircraft type assigments seem to follow the pattern typically seen at other European airlines who have significant domestic networks as well as an extensive network covering other cities in Europe (AF, AZ, BA, IB, and LH come to mind in addition to SK). In general, it seems that the larger aircraft in the fleets of such airlines are concentrated on their domestic networks while the smaller types are typically assigned primarily to international services that tend, overall, to generate less traffic than their domestic routes.
At least some of the 737-800s of SAS were delivered in an all-coach seating configuration with domestic routes in mind. From their current timetable, it is evident that 737-600, MD80, MD87 and MD90 types remain the workhorses of SK's network to European points outside of Scandinavia while the 737-700/800 types (along with MD80s) serve many of the domestic services within the three countries represented by SAS while the A321s are especially found on CPH-ARN/OSL services.
There are, of course, exceptions to the rule. During a recent trip to the FRA Visitors Terrace I saw the following aircraft of SAS within a 4 hour time period: Dash 8 Q400, MD80, MD90, 737-800, and A321. The reason for such unusual variety may be the Star Alliance partnership between SK and LH which makes FRA a virtual second European/intercontinental hub for SAS.