Why tractor has two large wheels?? plz ans scientifically?

Ketan is correct. It has to do with spreading the weight over the largest area on the ground, i.e., increasing the so-called footprint. That way, like snow shoes over the snow, the wider longer tire treads of the large rear tires are less likely to sink into the soft soil (e.g., mud) and get mired down. Also note these rear tire will have really deep treads; so as to grip the soil and give them better traction for pulling through the soft soil.