Apart from the look, is there any other reason why brake rotors on a mountain bike are bigger on the front than the rear?

Bigger = more power

More power at the front where most of the braking force is. Less power at the back so less likely to lock up as the wheel is unweighted.

Bigger radius means less torque for a given amount of lever force = MOAR POWAH!

Bigger circumference means more surface area to manage heat = LESS FAYDE!

On the rear, there is less traction, so the smaller disk doesn’t lock-up as easily.

You want more power on the front of the bike, the look is irrelevant. The front brake does most of the work and has to work with most weight over it (if your doing it right) so a bigger rotor gives you more power and more control