This is why I'm in favor of a combat system with four variables rather than two.
Defense = Avoidance skill
Attack = Accuracy skill
Damage = ....
Armor = Damage reduction
cloth/leather has higher defense and lower reduction, plate does the opposite in addition to slowing movement (without a mount)
Example using fake numbers:
Unit has 16 dexterity this provides a bonus of 16-10=6 let's pretend that 1 defense provides you with a 3% chance of avoiding an attack altogether. So naked, this agile unit has an 18% chance to avoid the attack of somebody with an equal attack bonus.
Now leather armor provides a 30% increase to your dexterity defense bonus, we'll round it to keep calculations simple. Now the unit has a defense of 8 or 24% chance to avoid equal skilled attack.
Leather armor also provides a meager damage reduction of two against slashing and bludgeoning, one against piercing.
Now take the same unit in plate. Full platemail will reduce your dexterity bonus to defense by 50% this unit has only a 9% chance of evading an equally skilled attacker.
It also cuts battlefield movement (dismounted) by 50%, to a minimum of 1.
The damage reduction on plate is 10 against slashing, 7 against bludgeoning and 6 against piercing.
Attacker has 16 strength, this gives him an attack skill of 6, and a damage bonus of 3.
He's using a broadsword which has a listed damage of 7 and gets a bonus of 3 from strength. So 10 possible damage.
He swings at the person wearing leather. With an attack skill of 6 and a defense skill of 6 the defender has a 24% chance to avoid the hit altogether, if that fails he will take 8 damage.
He swings at the person wearing plate, attack skill of 6 vs a defense skill of 3. 0% chance to dodge, because of the disparity created in the two skills a further damage bonus is applied since the attacker can more easily focus the assault in weakened areas or strike with more strength in the right locations. So the % disparity in attack vs defense skill, in this case 100%, is applied to the Str bonus, this provides +6 damage from str on top of the 7 the broadsword has baseline. 13 damage, 10 of which is absorbed. 3 damage total.
Obviously the numbers and such can be tweaked, but this gives a general idea of what I had in mind. Thoughts?