Generally, the numbers provided is not the main combat army.

When Napoleon moved into Russia, of about 500,000 soldiers, about a third to a quarter were French soldiers and these were the main soldiers that were used in battle (such as at Borodino) but the rest were used to protect supply lines. The main reason is that French supply lines would be vulnerable to Russian harassment.

Generally, the French worked with a rather limited supply train in order to lighten the load of the army and allow quicker movement in enemy territory. The idea was to have war pay for war, so food and other supplies were pulled from the land (such as foraging) and from local areas (which were often promised payment after peace). Since Russia was generally underdeveloped compared to Prussia or Austria, there was a greater dependence on the supply train, not just in respect to ammo (mainly gunpowder) but mainly on food and fodder (for animals). So, to ensure that these would make it to the main French army pushing it's way into Russia, the various allies pulled into the invasion were stationed at various points to protect and ensure a continued supply line.

Despite the in depth responses, I am not sure they are really responding to the question about why did Napoleon raise such a large army in the first place? He had a huge conflict going on in Spain as well as on again/off again stuff such as Walcheren in NEtherlands and stuff in Italy.

As to the issue of getting your enemy to fight. In the end the Russian army did stand and fight: at Smolensk and then at Borodino before Moscow was captured. Borodino I seem to recall was the bloodiest battle up to that point so it is hard to expect them to fight any harder.

So Napoleon did get the battle he wanted, but it turned out that Alexander did not see it as time to negotiate but preferred to scorch the earth and retreat from Moscow. Part of this seems to be that Napoleon expected the war to go like most wars in those days; a year or two of fighting and then make peace, adjust some borders and everyone goes home. But there has to be more to it than that. After all, Nap. had the example of the Spanish war which was dragging on. As well as conflicts between Turkey and Russia that went on.

He seems to have counted on his personal relationship with Alexander to allow the two to reconcile after one big battle. After the treaty of Tilsit in 1807 you may recall these two seemed to be on close terms. The famous meeting on the raft in the Nieman river which was very secret.

After the Erfurt conference in 1808 Alexander really did not see eye to eye with Napoleon and of course Talleyrand who was Napoleon's diplomat seems to have double crossed him as he felt Nap. was leading France to destruction. So if Nap. was counting on Alexander to back down after one battle he was mistaken.

That's about the best I can do. I think there is a lot of secret diplomacy that went on at this time, that we still do not know a lot about. Talleyrand in particular but others e.g. Fouche and his role etc. Also I really wonder about the personal relationship between these two men what really transpired on the raft?

I think yours is a really good question (why so large an army?) and I dont think I've ever read anything directly on point as to why such size in particular. Two best books I have read, and it was about a year ago for both are: Moscow 1812 by Adam Zamoyski, and Charles Esdaille's Napoleons Wars an international History.

Raising such massive army had three interrelated functions. One, as has been stressed in other responses the was the need for logistical support for such a large army. The Eylau/Friedland campaign familiarized Napoleon with the difficulties of operating within Eastern European terrain and although he won that campaign he was determined not to replicate the campaign. Secondly, Napoleon's fundamental strategy was to bring overwhelming force onto the enemy and crush him. Since he knew he had a large logistical tail, this meant the Grande Armee had to be larger than its predecessors. Finally, a massive army had a political purpose. Not only would it allegedly cow Alexander II, but it would allow Napoleon to keep tabs upon his less than enthusiastic allies. Napoleon in particular did not trust the Prussians and placed them in rear area duties (including guarding supply depots) and the flanks under the X Corps.

Although it is easy to critique Napoleon's strategy and conduct of the 1812 campaign, there was some merits to his plan. The Russian army's senior leadership was very divided as to what should be Russia's grand strategy. Minister of War Barclay de Tolly favored a Fabian strategy of a strong resistance at the frontier followed by a scorched earth retreat and counterattack. An offensive-orientated faction clustered around Prince Peter Bagration called for a preemptive attack into Poland followed by a war of liberation. Other offensive minded generals agreed with de Tolly's Fabian strategy, but argued that the scorched earth should be in Poland, not Russia. Alexander II complicated matters further by never committing to one plan or the other. Had Napoleon been able to bring the weight of the Grande Armee to bear on the Russian early in the campaign, the Russian armies might have been trapped out of position.

As it happened, the Russians made the best out of the strategic confusion of its upper echelons. Faced with overwhelming numbers, the Russian Army elected to retreat and enacted de Tolly's scorched earth out of necessity. But the offensive faction emerged out of the pattern of retreats with greater sway in the court and within the army.

The question is of course why couldn't Napoleon take advantage of this situation? Part of this answer goes back to the OP's original question: the Grande Armee was too big for Napoleon to wield. Very few of Napoleon's subordinates were capable of truly independent action (Davout and possibly Gouvian St.-Cyr being the exceptions in 1812). There is also some evidence that his staff system headed by Berthier was unable to simultaneously coordinate the large Grande Armee and translate Napoleon's orders effectively. When the frontal elements of the Grand Armee made contact with retreating Russian troops, the Grande Armee could only clumsily lumber towards it. Napoleon had thought he could destroy the Russian army within Lithuania, but the retreats prevented this.

Sources

Leggiere, Michael V. Napoleon and Berlin: The Franco-Prussian War in North Germany, 1813. Norman: University of Oklahoma Press, 2002.

Although not directly about 1812, Leggiere has some good insights into Prussia's contribution and the strains within Napoleonic decision-making.

Mikaberidze, Alexander. The Russian Officer Corps in the Revolutionary and Napoleonic Wars, 1792-1815. New York: Savas Beatie, 2005.

_. The Battle of Borodino: Napoleon against Kutuzov. Barnsley: Pen & Sword Military, 2007.

Mikaberidze has done excellent work in bringing together and translating Russian-language sources to fill out the Russian side of the equation in 1812.