The border did not use to be that way.

Here's an image showing some of the border changes in Norway

The counties of Jämtland, Härjedalen and Bohuslän, which are today part of Sweden used to be part of Norway, as were the parishes of Särna and Idre.

The King of Norway captured the independent peasant republic of Jämtland 1178, and extended his infulence over the hinterland east of Tröndelag in the following years. Back then, the influence of the King really only reached as far as he could project troops stronger than any rival Kingdom or any locals not wishing to be part of his Kingdom.

Norway is a small stretch of land because most of it is on the west side of the Scandian Fells - while no Alps, it is still a large hindrance to projecting power. At the border between Jämtland and Tröndelag, these mountains are easier to cross, as they are further south, which explains why the King of Norway ruled Särna, Idre, Jämtland and Härjedalen - it was easier and quicker to send troops by ship to Trondheim and march to Frösön or Sveg than it was to march by land from Sigtuna or Stockholm.

Nowadays, the border of Norway follows the Scandinavian Fells. Since we know that the border always followed these mountains (except when the aforementioned provinces were part of Norway) we can make a pretty exact border on maps. We know far less of many ancient and medieval states to draw their borders that exact.