Typically what I've seen that makes some sense to me in a real world setting, is that while wearing armor may make it a little harder to hit you in some regards, It's primarily reducing incoming damage.
So as you increase the amount of armor that you are wearing, you'll take less damage from incoming attack. At the same time though, the extra armor is stiff, and weighs a lot, so it will slow down your ability to move freely, which in turn, typically makes it easier to hit you since you can't dodge or parry as easily.
Now the ratio of damage reduction to dex penulty typically depends on what balance you're looking for, and how much value the stats have, but there's other things to consider for the bonus/penaulty values. Things like the material it's made out of (ie Iron chain vs Steel Chain vs Mithril Chain.) You also have the level of craftsmanship involved, where a highly skilled craftsman might be able to make a lighter piece of armor without neglicting protection.
That not even getting into enchantment spells that might let metal flex like cloth until it sustains a blow, similar to the Newtonian fluids.
