I don't believe anyone has posted anything that has definitively supported their 'belief' that a Kubota is inherently weaker because it does not carry the same weight as another tractor. Nor has anyone posted anything definitive to support otherwise. There has been some rather heated post on both sides of the argument. Some have stretched believability a lot further then others too. I really like the statement that decries Kubota are lighter to save on shipping costs. Absolutely ludicrous.
While I can honestly say I don’t know what specifications Kubota uses in their castings nor am I privy to the engineering used to design their tractors, I do know that Kubota is not suffering from a rash of failures due to the improper strength of the castings, frames, or axles. If Kubotas were falling apart as soon as the plow hit the soil or the disc ran into some clay, then I would be worried. But that isn’t what is happening.
Heavier can be stronger or it can be weaker but it will always weigh more. Until somebody gets a group of these tractors together, outfits them the same, puts the same implements on them, and then runs them side by side with increasing workloads until something breaks, its all just supposition. So, who is up for some destructive testing? Anyone wanna call Mythbusters?