Hi folks,
I just got my 6D last week (upgrade from the T2i/550D). It seems to have a lower exposure with the same settings when i do a comparison. And fairly significantly so.
I was doing a comparison with ISO whilst keeping the aperture and shutter speed constant. Of course, the 6D has less noise by far, but is also darker at every ISO.

The lens on the 550D is Sigma 30mm 1.4, compared with the Sigma 50mm 1.4 on the full frame. So surely the lens can be ruled out as the issue.

Is this a known difference? Or something i should be worried about?

Just a long shot - LCD brightness setting?

Or are these comparisons made in an editing system?

I noticed this too in comparison to the t2i. I ran this test a while back ago:

Canon 6D lowlight test - YouTube

The difference is only about a half-stop. Never bothers me since it's not the meter under exposing, it's just a little darker at the same settings as the t2i.

I'm glad I found this thread, and the video above is quite informative. I rented a 6D recently and found it was underexposing compared to my T2i. I wondered what I was doing differently. Eventually I figured out it was just the difference in cameras. I also have a T3i and T4i. The T3i exposes darker than the 6D and my T4i exposes about the same as the 6D.