Size lowering something's AC exists in 5e too newb, no where did I quote this was a mechanic purely from an older edition.
And an AC of 11 is EXTREMELY easy to hit. A rat, with an AC of 10 (again according to 5e) is only marginally easier to hit. It sure as shit isnt a 30-60% percentage as is displayed in the game. Make a lvl 4 fighter and roll the dice yourself, you'll be hitting the vast majority of the time.
Hello again! Apparently I needed to return here to point out if size, in 5th edition, was STILL linked to armor class would you care to explain why small creatures, who used to receive bonuses to their armor class in 3rd edition, no longer receive a bonus to it in 5th edition?
Perhaps you could also explain why the Enlarge/Reduce spell, a common size changer, has absolutely no effect on armor class? Could it be that size is NOT relative to armor class?
It's easy to make a mistake, but there's no need to fight people to the death when you do, just admit the mechanic of correlating size and armor class have been removed and move on to enjoy life.
