5th edition does not directly link size to armor class the same way previous editions did. An Ogre should have an AC of 11 because it's a lumbering brute... but that is only made up of a low dex and hide armor. A Fire Giant wears plate and has an AC of 18, nothing to do with size whatsoever. Both of these are also explaining how hitting a mob is one thing, but damaging it is another. A Fire Giant might be hit on a 16 but the blow just glances off the armor.
Remember to look at 5th edition and not try to use previous edition rules to form arguments.
