I found this on google answers, and I think it explains the physics of it...

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When a pitcher delivers the ball to the plate, it is coming (typically) along a line that extends from the center field wall (or very close to, depending on where the pitcher aims). That said, the maximum transfer of power occurs when the batter makes contact with the ball in such a way that it will travel directly back over the pitcher's head and sail towards straignt-away center field (the mid-point of his swing). If the batter makes contact before or after the mid-point, maximum power transfer will not occur and the ball will not travel as far as it could have. Thus, the center field fence would need to be longer than the others to try and take away some of the effects of physics when it comes to slamming home runs.

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Here's a link http://answers.google.com/answers/threadview/id/89869.html