Small Armor's looks and amount of armor points given. In Serious Sam: The Second Encounter and Serious Sam HD, it looks like medieval-style helmet and gives the player 5 armor points. In Serious Sam 2, it is a shield and gives the player 10 armor points. The Serious Sam 3 version gives the player 10 armor points and is a helmet In all of the games it appears in, it will not go past the “normal” maximum armor limit (100 in SS1, SSHD and SS3, and 200 in SS2).
In The Second Encounter, Small Armor is rare and only appears in a few locations. It seems to exist to give the player some armor points so that they can get to the 100 armor point limit without picking up a stronger set of armor. This can be useful if the player has a large amount of armor points and doesn't want to use up a larger set of armor in order to get to the 100 armor points cap.
In Serious Sam 2, Small Armor is much more common and is often hidden in crates. This makes it much more useful, as their abundance makes sure that the player will almost always have at least a few armor points, no matter where they are.
In every game it appears in, Small Armor is never booby trapped.