I could be wrong, but have been around tractors all my life, and to me the main reason for the size difference lies in the NAME...TRACTOR. When they were originally invented as steam engines, the only way the power could be put to use was to have large rear wheels and tires that could handle the power produced by the engines. The immense weight also required a wide tire tread to keep from sinking into the ground. There are a lot of four-wheel or all-wheel drive tractors produced that have identical sized front and rear wheel sizes, which to me makes sense but the vast majority of ride on mowers use two wheel drive, therefore the larger rear wheels and tires.
Remember...there are no stupid questions, only stupid answers...:laughing: