Why do tractors have big wheels in the rear part than at the front?

Contrary to popular belief, large-diameter drive tires actually provide less torque and pulling power because the axle has less leverage, and the mass of the tire is greater, which requires more power to overcome its inertia.
But large tires have greater surface area and contact with the ground, increasing their traction. Furthermore, because of their increased surface area, wide tires are less likely to sink into soft ground.
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Small Tractors