Funny one. Because it works both ways depending on what you want.
The rear gets more weight, so bigger is better in terms of taking the bumps and while you tend to see and dodge things the front may hit, the rear often smacks whatever is in the way.

The rear transmits the power, so smaller is better in terms of power transmission. So for a sprint - smaller is better.

In the end most folks use the same size. The rear will wear faster.