Why front tires are larger in combine?

They compensate for the smaller rear tires. The rear tires have to be small, in order to facilitate turning. The larger tires are used to drive the vehicle, so they need to have a large cross section to distribute weight evenly enough in order to prevent the vehicle from sinking into the ground. The large diameter of the tires also serves two functions - it helps keep the cab elevated above the radius of the head, and also keeps the vehicle geared low enough to deliver ample drive torque.