I would not presume to be knowledgeable as to how historically accurate the in-game depiction of Soviet and Finnish military strength vis-a-vis each other is. However, what this thread describes for the Winter War Scenario is quite different from how the Winter War unfolds in the GC. Specifically, in my experience, it is impossible to win the Winter War as Finland. I mean "victory" in the loosest possible sense, such as permanently holding the Soviets at the border, at least until Germany declares war on them, if they refuse to make peace.
The problem with that is Kajaani. Because this province can be attacked from 3 Soviet provinces, it is the weak point. Noticing this, I stacked up more divisions there than on the rest of the border - around 20+. I built fortifications as well. This worked fine for a few months. Then the Soviets attacked from those three provinces with around 50 divisions.
I've only played as Finland that one time, but every time the Winter War occurs while I'm playing another country, the same thing happens. The Soviets break through at Kajaani, and since that province borders many Finnish interior provinces, it is impossible for Finland to defend all of them.
Although, in hindsight, it does seem more sensible to attack through central Finland as the AI always does...it has to make you wonder why the IRL Soviets spent more effort attacking the fortified Viipuri region instead. It's even more ironic when you consider that the main Soviet offensive into central Finland ended in one of the Finns' greatest victories at the Battle of Suomussalmi.
Long story short, in the GC, it seems to be impossible to "win" as Finland without allies and at Normal difficulty (or higher). The only ways to win that I can think of are being in an alliance or lowering the difficulty level. Annexing other countries before the war might help, but I doubt it.