Hi. I'm sure Finnish members can provide more information, but here's few links I've managed to find about this topic.
http://www.mil.fi/perustietoa/talvisota_eng/ (Timetable of this war)
http://www.winterwar.com/ (Finnish site containing general information).
I have quite a lot of links and sources (mainly, History books), but they are in Russian, and you don't have time to translate all this text, so a few main points.
Taken from
Winter war from Wiki in Russian :
At the beginning of the war, here's how forces' ratio looked like.
Finnish Army Red Army Ratio
Divisions 14 24 1:1,7
Men 265 000 425 640 1:1,6
Guns and mortars534 2 876 1:5,4
Tanks 26 2289 1:88
Aircrafts 270 2 446 1:9,1
As you see, Soviets had huge advantages in tanks, airplanes and artillery, but not in grunts. This was due to Stalin's decision to send to war armies only from one military district - he thought it was enough to win the war. It wasn't. When attacking, one must have ratio of 3:1, when assaulting - 5:1 in order to win. At the first phase of war Soviets did not have such ratio.
Secondly, Soviet high command sucked - they were sending forces in echelons and not in big fist. That is, [just to illustrate] a company assaults Finnish positions and gets wiped out by MG fire from a flanking and well hidden bunker. Then, the second company is being send to do just the same. Then the third etc. As I read from memoirs, little was made in the beginning part of the war to locate those flanking bunkers. Then, after some fighting, it was obvious that something went wrong.
This is when Soviet forces were reinforced and measures were taken to locate and destroy the bunkers (either by huuge piles of TNT or by heavy artillery fire - Soviet divisions were reinforced by 81 guns of 203mm, 234mm, 280mm - for a long period). That's is second period of war and it's far from catastrophic, if one is looking from Soviet point of view.
Also, very important topic is.. I believe it's called "motti" in Finnish? Well, those are kessels - some big Soviet forces were surrounded. And they acted very, very passively due to their commanders' inexperience. They could try to break out, or at least do something to help themselves in their situation, but this wasn't done - they just did nothing in order to not be killed by Finnish soldiers and cold weather.
BTW, about the weather conditions. It's a well known myth that Soviet troops struggled in the thick snow in very cold temperature. But in reality, until до 20th of December, temperature on Karelian Istmush varied vrom +2 to -7. Until the 1 of January the minimal temperature was -23. A really cold weather strated in the second half of the January (up to -40C), but there was no active actions at that time. There was no thick snow until the January - for example, at 15th of December snow thickness was 10-15 cm. And it was really thicker when Soviet troops operated a lot better in the last phase of war.