very slightly, at the neck/shoulder...case is thicker on the 5.56 mil as well.
Internal. The key word is internal. If the typical loadings are used then the shooter has no worry using either brass for any role. The biggest difference is the rigidity of the military brass (5.56 NATO), it needs to be slightly more stout in order to be used and abused in military rifles and machine guns. A reloader typically can not get as may uses out of 5.56 NATO brass simply because of case head separation issues. Commercial .223 Rem brass is softer and is thinner per say, and is normally used out of bolt action guns which have slightly "tighter" chambers. However, it is 6 one way and a half dozen the other because eventually both kinds of brass will have issues. I simply separate brass by commercial and military and load accordingly, which for me typically means -3 to -%10 of the listed "max" loadings. If I'm pushing the pressure a bit high for purposes of velocity, I only so so with the Savage 112FV. The AR15 gets the typical military recipe.