Can confirm firsthand that you don't even have to do planetary arc to stay on level. I created a new Commando to test just this. She just finished Hoth, only does class quests, not even bonus quests (unless enough mobs are standing between her and her goal to accidentally complete it), only uses XP boosts she gets as rewards and she's still always one or two levels higher than most mobs.

Perhaps it's the Recuperate animation?

What I sometimes do when I'm out with my Operative and I see someone having a hard time, is heal them. Likewise, if I have my Vanguard out, I may taunt the toughest mob and just stand there taking the hits. I would typically not steal someone's kill though. For some reason I think that's a little obtrusive. And of course, I buff every ftiendly player I meet.

In fact, you can change your companion's class -- within limits. Most (if not all) companions have multiple stances, for instance tanking or dps stance, or healing or dps. Open your companion's skill bar (I use ~, not sure if that's the default hotkey) and click the stance you want to use. Furthermore, by right-clicking, you can toggle which abilities you want him to use in combat. For instance, I can make my Trooper's ranged tank function as a dps by changing his "generate more threat" stance to "do more damage" and by deactivating his threat generating abilities. Of course, since he'll be using only a few of his skills now, he won't perform as good as my real dps companions. But it's a nice workaround if you haven't unlocked all your companions yet.