Traditional Cat machines are underpowered for their weight (or heavy for their power, I guess it depends on how you choose to look at it)    Our 1550 Case has the power of a 6 but weighs less then a 5.  

Then there are the times when they built two different tractors with the same base model...D6M is a lot different then D6R.  6M is more like the power/weight relationship most other mfgs build to.