I know that this thread is 4 months old but I have a old 1980's T1400 and I like the lawn tractor, I can also support the fact that those machines were built heavy. In the spring I can not use it some of the poor draining areas of my yard. It will sink right to the axles.
My 2003 JD L120 just floats over those wet areas. Of course the Kubota is 150 pounds heavier and on a smaller tires might be some of the issue