Metallurgy, machining tolerance, heat treating, etc... all are parts of it.
Again, it all comes down to how much pressure a given design can deal with. 5.56 chambered pencil barrels will have the looser tolerances. They also lack accuracy because of it. The same pencil profile in .223 match chambering would indeed come apart under sustained 5.56 diet. The same .223 chambering with better steel and a bull barrel profile can eat 5.56 all day long, even if you end up ironing a few case heads.
As posted previously, a 5.56 round designed for the longer leade of the 5.56 chamber will often be jammed into the rifling of a firearm chambered for the .223, causing increases in pressures of up to 20,000 psi, making every shot a proof load.
Thanks for the discussion.