Sith Marauders do wear heavy armour but not like a Bounty Hunter type armour it still looks like a cloak. As for helmets I'm not sure only saw hoods in the beta weekend but it think there are some different things for them.
Are you sure they wear heavy armor? Pretty sure Sents only wear Medium and Marauder is the mirror of them.
EDIT: Confirmed it on TORHead. Marauders only wear Medium