Yesterday when I was playing online with my friend there was a guy on the security team using the Gotlung Minigun, but he was a light body type. I know it wasn't a bot because he had a wierd gamertag, a couple of x's and o's, not like the bots normal names. Plus he was in my recent players list.

How the hell does that happen? I know light types can't even use AR's, how did he have a minigun? Cheating?

I have to admit, I've also seen the same thing you're talking about
But when I looked closely at the character, I could tell it wasn't a light weight type
Reason? Light weight types are the thinest body types you'll ever see in Brink, Heavy body types actually don't look that huge and buff in game
The difference you need to see is their shoulders, Heavy body type's shoulders are broad while light weight body types have thin shoulders

That sounds a lot like you spotted a heavy operative in a light's disguise...

He was shooting at my friend though, I thought you give up your disguise once you pull up your gun.

It could be heavy I guess, but he looked pretty small.

OP is right. There's a glitch if you make your body heavy, change weapon to minigun. Back to medium/ light. You will start off in game with a minigun.