With a tanning developer there is very little migration of silver halide during development because early in development a hard relief image is formed. This results in more precise reduction that minimizes infectious development (spreading of silver development beyond the exact image boundaries).
You can see the result if you make two negatives of a strongly back lighted scene, say a tree in the winter with no leaves against the sun. Develop one of the negative in a traditional developer, the other in a pyro type developer (PMK, Pyrocat, etc.) and compare the results. Chances are there will be a lot less infectious development with the pyro developed negative.
Some years ago I carried out some resolution tests comparing two pyro type developers (PMK and Pyrocat-HD) with two traditional developers (D76 and Xtol). With every film tested the pyro developers gave about 15% higher resolution. The statement that Pyro developers are sharper is not a myth, but a fact that can be tested and proven.
Sandy King