Assuming the big and small tractors have similar features and complexity, it is the same number of parts to design, fabricate, and assemble.
The cost difference is going to be in the increased materials. if they are buying the steel for $0.25 per pound, and one tractor weighs 1000 lbs more than the other, its only $250 more in cost.
I took a tour of the Ford (truck) assembly plant in St. Paul once. It made me realize that the difference between the smallest, stripped-down Ranger model and the biggest, decked-out F-250 that they built there was hardly anything. They were built randomly inter-mixed on the same assembly line! As each truck would roll by, it would get big or small wheels, big or small engine, big or small seats, more or less chrome, simple stickers, or plastic badges, etc. Exactly the same number of people, nearly the same number of parts, and exactly the same amount time was required to build each truck, no matter what model!
Based on the fact that most of the engineering and investment was in building the factory to build the trucks, I can't imagine it cost them more than 10% difference to build the biggest vs. smallest model.
- Rick