Okay, I read through all the replies in this thread, and I want to comment on a couple.
First, Russia has indeed had its tail whupped many times. The Poles conquered much of Russia. The Turks beat on them, until General Suvorov came along. They fared poorly against the French in the late 1790's, again until Suvorov came along. Nappy whipped them at Austerlitz, and drove them back out of Austria. Later, after beatin' the bejeezus out of the Prussians, they also beat the Russian/Prussian army at Eylau, thus forcing the treaty of Tilsit upon Russia. Russia was forced to forgo trade with Britain, and was technically an ally of France. Germany defeated Russia utterly in WWI. Russia submitted, then went into complete civil disorder, actually splitting into two, a la CivII.
France lost to Russia in 1812 because of the weather. Nappy started the campaign late, because he wanted the wheat and such to be ripening in the fields, so his horses could readily forage, but there was an extra 3 week period of wet weather, which meant that forage was scarce. The supply problem for his horses (and men) wasn't alleviated until August. Russia and France fought a battle near Smolensk, in which the Poles made some serious tactical mistakes and sufferd high losses, due to the overexuberance of their commander, Ponyatovski. Then they fought at Borodino in early September. Nappy chose the wrong tactical approach. He and Davout argued about it beforehand. Davout recommended a flanking move, but Nappy chose a direct assault. Davout was right, and Nappy never used him afterwards in any crucial role. His absence at Quatre Bras and Waterloo led directly to Nappy's ultimate defeat. Nappy went into Russia with 650k men, not 800k. About 100k of these were Prussians and Austrians, who actually added nothing to the attack, as they functioned merely as show forces. The French got out with 60k men, not 10k. They didn't really take tremendous losses getting out. It was merely the terribleness of the suffering. They lost far more going in, as desertion and other attrition wore them down far more than the enemy ever did.
In WWII, Germany came very, very close to taking the whole ball of wax. Strategic blunders, especially economically, and the effect of all their private empires defeated them.
So, what does this mean in Civ terms? First, yes, Russia is huge, but it is incredibly underdeveloped. It is why these littler civs have been able to wail on them, especially when the Russians ventured outside their own borders. When invaded, the lack of roads and infrastructure made the captured cities worthless. You couldn't just take the cities and roll on down to the new border. Further, the lack of city improvements means resistance, unrest, and flipping are greater threats, slowing the attacker. The new cities don't integrate well into the conquering culture. By the time the attacker gets deep into Russia, he's a long way from his reinforcements, and much of his invading force is tied up in partisan warfare (city garrisons to keep 'em content). The lack of decent road means it is easy to interfere with the arrival of said reinforcements. Further, the Cossacks, with their speed, become deadly in this environment. This was true in Napoleonic times, as well as WWII. Read any German's account of the Eastern Front, and you will see his deep concern when talking about the way the Russians used this anachronistic force. However, once out of Russia, and into road-rich environments, the Cossacks become far less effective, as it lost its speed advantage in enemy territory.
Now, factor in the government types the Russians. In Napoleonic times, the Russians used a Feudal government, with all that entails. In WWI, they were a Monarchy, but Monarchies don't work well in huge civs. Russia was actually defeated by war-weariness. They were literally rioting throughout the kingdom, so they sued for peace, changed governments, went into anarchy, became communist, and fought a civil war. The lost territory eventually became a renascent Poland after the defeat of Germany. In WWII, they were, of course, Communist, with all that that entails: huge garrisons, not much infrastructure that is useful to an attacker. Same result in WWII as in Napoleonic times. So, size is important, but so is efficiency. Communism allowed the Russians to be somewhat efficient, without having the infrastucture of a modern state. The fascists were more inefficient, but Germany was enjoying a Golden Age. Italy was merely pathetic, as it was inefficient and long past its GA. The minor allies were all small Monarchies, and tech-backwards. The Western Allies were wealthy democracies which enjoyed War Happiness. With their unlimited resources, they bombed Japan into submission, turning Japan into "a nation without cities". They built massive navies, and slews of bombers. The ground forces were relatively limited. The US took a "90-division gamble", deciding to concentrate on other branches, and tech research. The UK had about another 20 divisions, choosing the same emphasis. Compare those numbers to Germany, which mobilized some 250 divisions, 50 of them armored. Now try Russia, with 400+ divisions, with a little lower ratio of armored forces. The democracies chose to buy all the really expensive things, like Bombers, Battleships, Carriers, Atomic Bombs, and rushing city improvements.
In the end, Russia came out the big winner, because of Roosevelt's foreign affairs incompetence. He just couldn't understand that Russians don't understand noblesse oblige, and he sold Eastern Europe down the river at yalta in Feb '45. However, the Russian communist empire eventually collapsed, due to lack of infrastructue. They did a completely AI thing, and built military until it couldn't afford anymore, and lost their gains due to culture flip.
So, bigger is better, but strategy plays a big part, too. Now, Russia is much reduced, and suffers all kinds of unhappiness. The U.S. is the dominant player in the game, right now. China and India are merely distant seconds, only in the running because of their huge populaces. The Euros are reduced to just a few cities each.
! But ALWAYS attack, try never to defend. Like the military advisor tells you to.
