Extra Small Armor's looks and given armor points depends on the game. In Serious Sam 1, it gives the player one armor point when picked up and looks like a red and yellow shield. In Serious Sam 2, the Extra Small Armor is a large blue and yellow helmet with designs on it that gives the player 5 armor points when picked up. The Serious Sam 3: BFE version is a plate of armor in a sack and gives one armor point when picked up. The Serious Sam: Next Encounter version looks like the Serious Sam 1 version, but gives the player 5 armor points when picked up.
The Serious Sam 1 and Serious Sam 3 versions of this item will let the player exceed the normal 100 armor point cap and go up to 200 armor points.
In Serious Sam 1, a single Extra Small Armor is often used as a booby trap to spawn in more enemies. Groups of Extra Small Armor can be safe depending on the context. If one appears after you pick up another one, it's usually leading to a trap where an enemy spawns at the end of the Extra Small Armor chain. If they're just sitting around, they usually aren't a trap.