This is a true case of "no pain, no gain." Your Armor skills (light, medium, heavy, or un-) will only generate skill increase points when your opponent successfully hits you. The exception to this is your Block skill, which generates skill increase points when you successfully block the attack (and thus take no damage from it).
I do not have any hard and fast data to support this next part, so take it for what it's worth.
Bethesda appears to have implemented a "hit location" function. For all of you old pencil-and-paper gamers, you know what I mean. For everyone else, it's a random function that decides whether you were hit in the head, arm, leg, toe, etc. This is why, when you equip an armorer's hammer, you'll see that your cuirass and boots are in need of repair, but everything else is in pristine condition.
The logical conclusion from this odd bit of data is that you MIGHT be able to develop several different armor skills at the same time by wearing a variety of armor types on different parts of your body. I do not know this for a fact and could very well be completely wrong, but it's something worth considering.
Back to the facts. The defining difference between Light, Medium and Heavy Armor is its weight, not the material of the armor.
|Piece||Body Part Protected||Light||Medium||Heavy|
|Helmet||Hair (shades of "Spaceballs")
|≤ 3.0 pounds||3.1 to 4.5 pounds||≥ 4.6 pounds|
|Cuirass||Chest||≤ 18.0 pounds||18.1 to 27.0 pounds||≥ 27.1 pounds|
|Pauldron||Upper Arm||≤ 6.0 pounds||6.1 to 9.0 pounds||≥ 9.1 pounds|
|Greaves||Groin/Upper Leg||≤ 9.0 pounds||9.1 to 13.5 pounds||≥ 13.6 pounds|
|Boots||Foot/Ankle||≤ 12.0 pounds||12.1 to 18.0 pounds||≥ 18.1pounds|
|Gauntlet||Hand/Wrist||≤ 3.0 pounds||3.1 to 4.5 pounds||≥ 4.6 pounds|
|Bracer||Wrist||≤ 3.0 pounds||3.1 to 4.5 pounds||≥ 4.6 pounds|
Bound Armor (summoned through Conjuration spells) has no weight, so it probably qualifies as Unarmored or Light Armor (0.0 weight comes up as Light) in spite of the fact that it's essentially Daedric armor and provides some heavy duty protections. Just FYI, there are no Bound Greaves in the game.