Tractor Design

Because that's where the drive is.

That is usually the case with a 2-wheel drive tractor, but not a 4-wheel drive, because in a 2x4 () the larger fronts are more expensive, provide less visibility, more difficult to turn (especially in the early days of non-power assist), and can provide a tighter turning radius; conversely, the larger rear tires give better traction and also provides a "cheap" gear reduction.

I agree with everything you said except for the "cheap gear reduction" part. This is not true. The larger tires actually give the tractor a higher final drive speed since the circumference is larger.

In most cases the front tires are smaller when the tractor is 2wd. However, there are 4wd tractors with smaller front tires... these are sometimes called front wheel assist since they don't pull nearly the same amount as the rears. Once you get into the very large and powerful 4wd units, they virtually all have same sized tires in front and back for max pulling power. Often times they are in dual configurations or even triple!

It's got something to do with the steering track rod needing to pass under the engine rather than through it.