Citrus fruit are good for you, but they contain quite alot of fructose, which is basically a sugar. So if you are eating them to excess, and not burning off the calories, then yes, you could put on weight. Good news is at least they dont have much fat in them to burn off as well!
I don't think not exercising causes you to put weight on. I would say that not exercising in and of itself would cause weight loss, with muscle degenerating into fat etc. (so long as you go poopoo).