per Hornady… “The 5.56 NATO chamber has a .125” longer throat. This allows approximately one more grain of powder to be loaded into a 5.56 NATO cartridge; this is what gives it higher performance than its 223 Remington cousin.”
https://www.hornadyle.com/resources/le-faq/what-is-the-difference-between-556-nato-and-223-rem-ammunition
Yeah...cartridge dimensions are the same, as I posted. Powder charges and chamber dimensions vary. There may be about a 1% difference in case capacity, due to thicker brass used in 5.56.