Editing RPG System Symmetries (section)

===Relative Attributes===
In systems that define contested actions, characters with the same relative skill should have the same chances against each other. Note that relative in this case could refer to either a linear scale, a logarithmic scale or any other appropriate scale. So for one system, characters with +10 skill should always have a 90/10 chance, while in another system characters with x2 skill should always have a 75/25 chance. The important point is that this holds for some consistent scale.

Most systems are good about this as it's usually a prominent design decision that's fixed early on.

This can be extended to a broader range of results than just success/failure, though that may not always be appropriate. (Imagine the outcomes of a contest of strength between two regular guys and between two copies of the Hulk. Success/failure rates may be the same, but critical rates may not be.)