hahaha my mamma had a cat once and he was sitting on the bed, then suddenly he rolled and fell off the edge, it was about 5 years ago but I still remember the look on his face as he realised and frantically tried to grab the bed.

But aaanyway, your average kitty usually weights between 3 and 7 kilos (females being less, and some large breeds such as Maine Coons and Norwegian Forests weighing up to 9 or 10 kilos). So 5 and a half doesn't sound too bad, definitely not obese. if you have any concerns about his weight get your vet to look at him, and if you put him on a diet to make him lose weight I would also recommend discussing that with your vet.
In other posts I have talked a LOT about pet foods, so I don't want to repeat myself for everyone to have to listen to again, but there are premium foods made for weight control. If he is a shorthair, try running your hands down his body. If you can still feel some rids, then his weight should be okay. If you have difficulty feeling them, then he may need to lose a few.