MARS suits is a type of body armor.
While it is arguable that personal armor itself could be considered light powered armor, it is still no heavier than conventional tactical protection worn by infantry around the world. Power Armor proper was a full-body enclosure with no exposed body glove or other vulnerable sections, capable of extended and augmented operations under any environmental condition with optimal protection from plasma and conventional weaponry. The MARS version is also uniquely equipped with anti gravity propulsion systems similar to those found on Cyberdisks or the Roaming Susan UAV.
They are hulking gunmetal grey constructs with open clamshell sections around the upper legs and torso to accommodate an occupant who was as much pilot as soldier. Inside, it is very similar to the personal armor, with a visible layer of spongy soft, body conforming material. This was used to monitor physiological status, maintain body temperature and moisture, and would form a sealant in case of catastrophic breech or failure preventing terminal blood loss or shock. Unlike personal armor, the powered armor is all one solid unit, and couldn't be completely disassembled into separate sections for arms, legs, torso, etc.
This was why XCOM personnel wore little beneath their armor.