Braking is only one of several factors affecting tire and wheel design. The rear wheer, for one thing, is the drive wheel. That means not only does it experience far more stress at all times, but maintaining a larger contact area with the ground reduces the chance of breaking traction.
As to braking: while it's true that the front wheel ends up handling a lot of the stopping power due to basic mechanical physics, it's also true that the rear brake is critical to avoiding a somersault crash.
The rear wheel handles far more weight at all times other than a hard brake. Most of the bike & operator mass is nearer the rear wheel than the front one.