Originally Posted by Retro Grouch
Campy is a little different. Medium cage Campy derailleurs will handle a slightly larger rear cog as well as wrap up more chain than the comperable short cage version.
The campy medium is more in the spec than anything. The short will handle the large 29 just as well in almost every current application.I have some older frames with with a short hanger where that might not be the case.