Please explain semi-auto barrel dimensions

[Bigfoot]

I've always wondered why AR barrels were stepped like they are and this caught my eye on the Kel-Tec RFB flyer:

"BALLISTIC

The RFB barrel acts as a spine in the rifle. The compact
receiver and tilting bolt close the breech. The sighting
system is solidly attached directly to the barrel. Forward of
the sights the barrel is free floating. Close to the chamber the
gas pressure determines the dimensions of the barrel.
Further forward, rigidity and mass are more pronounced for
the sake of accuracy."

http://www.kel-tec-cnc.com/images/downloads/RFB_Flyer_SHOT_2007_web.pdf

I assume they mean exterior dimensions, why?

The barrel on the RFB is thick/heavy because it acts as the structure. If the barrel close to the chamber is turned down to remove weight will it affect functioning? Please forget rigidity for the sake of this question.

 

[Wes Janson]

The barrel is turned down ahead of the chamber, between the chamber and the gas port. I'm guessing that the reason for this is to reduce weight in a section of the rifle where stiffness isn't as critical for accuracy (because of the twin mounting points). Basically it's done for weight savings.