We've debated this at length over in another forum. But here's the gist of it:

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The imbalance in the Force is Sidious, not the Jedi. A properly trained Jedi acts with the balance of the Force in mind. Ultimately selfless and the means justify the end. The Sith care nothing about the balance, using the Force to only augment themselves. Ultimately selfish and the ends justify the means.

Or, in other words: A properly trained Jedi is mindful of keeping the Force in balance. He does not affect the balance of the Force. We're not talking about the number of Force users on each side, but how the Force is used.