When the borders were drawn after the Winter War, does anyone know why they were set where they are?
Because for many years I've noticed that the section of the border running from the Gulf of Finland to NE of Joensuu is roughly parallel to the Finnish coast between Vaasa & Oulu, and that if you rotate a map of Finland so that SW is at the bottom, the southern part of Finland becomes symmetrical in shape. Any idea why this is so?
The borders of Finland changed twice, once after the Winter War and after the Continuation War/WW2.
That territory in the south-west is Karelia, and the reason for the Winter War was a Soviet demand for land on the Karelian Isthmus in return for giving the provinces of Repola and Porajärvi to Finland. The Soviets stated they wanted the isthmus for defensive purposes to protect Leningrad, but the Finns refused since they didn't trust the Soviets and the land in question had the Mannerheim defensive line so if that was ceded then Finland would be left vulnerable and the gates would be open for future attack.
Now, after Finland lost the winter war, the Soviets annexed more than what they had originally asked for in the ultimatum for the isthmus. The border was drawn in the south-west on the recommendation of Soviet defense strategists who wanted to avoid natural obstacles like Lake Ladoga, Bay of Viipuri, or the swamps around Saimaa. This new border left Finland with little natural defense barriers on their side.
After the Continuation War/WW2, the border in the south-west remained the same, and the only difference was Petsamo peninsula in the Arctic was given to the Soviets.