If you didn’t mess with resolution/aspect ratio settings before then I would try switching the aspect ratio of your TV. It’s possible you have it on some sort of stretch mode.
The reason I say this is because the 2.6 image by default displayed the retroarch NES emulators at a 4:3 ratio on my 16:9 TV. It looks great and I didn’t have to change a thing. When you start messing with the aspect ratio correction in retroarch the output image gets fuzzier in my experience.
Well, my RPi B+ is hooked to my computer’s lcd via DVI. If I start a snes game via pisnes, the game doesn’t take up the full screen. There are black bars at either size and the game is at the correct size.
However, nes games take up the full screen. It doesn’t look horrible, but I prefer to view it the way it was originally intended. Sorry, I’m just a little picky in that regard.