I currently use a Canon 550D and am considering moving to a 6D. One of the often-mentioned benefits of a full-frame camera is the ability to produce large prints. Given that the jpegs from the 6D are only a few hundred pixels taller/wider than those from my 550D, how can this be? Or do you have to shoot in RAW and convert it to a jpeg that has larger dimensions than the SOOC jpegs? If so, theoretically the same could be achieved with 550D.
Theoretical discussions of print size per megapixel don't take the real world into account. But once you have a sufficient amount, megapixels aren't really the biggest factor in image detail. Enlarging a print from the recording medium (film or sensor) by the smallest amount is an inherent advantage — everything is multiplied by a smaller factor. And full-frame has that advantage.
Of course, there's some line where older full-frame technology is beaten by newer smaller sensors, but for roughly equivalent technology, lower enlargement is better.