It's not really the bullet diameter that makes the difference... afterall, the Ruger Blackhawk convertible can fire both calibers, but you have to swap cylinders to be able to do it. Of course, I've heard of erratic or unpredictable accuracy from the 9mm out of a Ruger. But yes... it is the chamber that won't let you run 9mm in the .357 Mag Contender barrel.
Worth noting is that T/C either does or has offered both .357 Mag and 9mm barrels for the Contender, so they are out there. Of course, anyone familiar with the Contender legend knows that you can have a barrel custom made in almost any caliber on earth. And if it's too big or punishing for a Contender, the Encore can handle it.
I love my Contender, but I've never understood why they even produced a barrel in 9mm. Now I'm not bashing 9mm, it's definitely got it's place and history in modern times, but in a long barreled single shot-- why? Everything in the world that is good about a 9mm has to do with do a (relatively) lot in a small space, allowing a semi-auto pistol to carry a heap of rounds and in particular, the overall small size of the round allows them to build small pistols. I just don't get 9mm in a 10, 12 or 14 inch single shot handgun.
Does anyone have any insight? Or was this simply, "people will buy a few, so we'll make some."