I'd suggeest it has to do with the design of the surfaces that are to be lubricated. Diesels that use cam-lobe driven unit injectors tend to extert enormous stresses on lubricated surfaces, which in turn, causes the oil to shear. Bearing loadings can also vary as a function of the bearing design. Also smaller engines will tend to need to run hotter to achieve equivilant power to a larger engine, which in turn places additional stress on a lubricant. Conversely, larger engines tend to produce more blowby simply because they have more cylinders and are built to less stringent tolerances.
Some engines have very efficient cooling systems, with oil coolers and oil/water heat exchangers, while others do not. I'd personally guess that full-flow filtration actually has very little to do with how well the oil stands up.
