Pyro/Cat development tans (hardens) the emulsion. There is less silver developed and this is "made up for" by the stain which increases density proportionately with the amount of silver developed (ie the amount of exposure). This does not necessarily result in highlight compression in the negative, although there can be a compensation effect when printing on VC papers.
Originally Posted by Alan Johnson
One thing tanning does not do, is prevent silver migration, because silver does not migrate to begin with. That is a non issue with modern hardened films.