It is quite simple. A barrel designed to withstand the pressure of a 3 1/2" shell will have a cerain wall thickness at various points. If you reem out a 3" barrel in order to lenghten its chamber a half an inch, you must consider if the walls of its barrel will be thick enought to withstand barrel pressures.
Not only might a 3-1/2" shell raise barrel pressures above what a 3" barrel can safely take, the wall of the barrel will now be thinner due to it having been reemed and the outer wall of the barrel tapering down from breech to muzzle.
Also consider any thinning of the barrel wall due to lentheninf of the forcing cone.