Well the brake hub has to be larger than the sprocket hub so it can slide over it to reach the other side of the axle. It being larger, requires a larger seal. The sprocket hub is smaller so it doesn't slide past the center of the axle.
Seems like they could have made the seals the same if they wanted to, but they didn't.
I have no idea if this is accurate, just the way I (and budweiser) see it.
EDIT: The hubs are different sizes and the seals ride on the hubs, so they have to be different sizes.