This magic armor makes up the armored division of the martial Milites Empire. Thanks to its Crystal, the Empire is able to mass produce the heavy armor - which it will use to declare its war on all Orience, beginning with Rubrum...
Colossi are a family of enemies. Gigantic sword-wielding forgekin often employed by the Garlean Empire, they serve as the game's analogues to Iron Giants, including members named specifically as such. Given that members of the family can be found in Allagan facilities, it is possible the Garleans salvaged the technology from the Allagan Empire.
Colossus appears as an enemy in Battle Music Sequences.
The original meaning of the word colossus is an exceptionally large statue.