I work in a big company as one of the more experienced guys in the department. There's definitely more work to do than I can do. Some coworkers work hard to get rid of as much work as possible by putting it on other peoples desks. In the mean time I've become efficient in being assertive and saying no to anything I don't like doing (and which is not my responsibility to do).

So you say you don't have much to do at work? Are others complaining about that? If not, good for you. It's better to do less, but do it well, than the other way around.