Jim,
The magnum action is longer than the "medium action" that they chamber the .300 Win. for. It also has the "drop box" extended magazine. I don't have a medium to measure, so I can't say if the magnum is actually larger diameter, but I suspect that it is. It is a true magnum sized action to handle the diameter, and length of the Rigby cartridges.
Hope this helps answer your question!