Heavier Robots are harder for an opponent robot to pull over the line.

If your robot use gears to make its wheels turn more slowly, this can also make your robot more difficult to pull over the line; to do this you would put a small gear on the motor axle, and a larger gear of the wheel axle.

Sometimes using more than one tyre on an axle can make your robot more difficult to pull over the line; but this will depend on whether your competition is being held on a table-top or carpet.

Sometimes more motors can be an advantage.

Have you talked to your teacher/mentor about whether your robot needs wheels at all?

Robot Tugs-of War on the Web?

You might also get some ideas from seeing robot Tugs of War on YouTube:-

http://au.youtube.com/watch?v=FCNqOeCXPeU 

https://www.youtube.com/watch?v=uztX71zPZmw