Dry food is high carb, and most has a pretty low nutritional value (including Science Diet, though I am not sure about Royal Canin). If it were me, I would purchase a higher quality dry food for nibbling on during the day (there are several threads about this here on the forums), and I would also start feeding a good quality wet food. Science Diet is expensive, but it really isnt the best food you can get for that price.
Our 2 boys currently get 1 cup of Science Diet T/D, which is larger kibble size and good for their teeth (yeah, I know, I just said it isn't the best,and I am looking for something better to switch them too that still has the larger kibble size!).
In addition to that they each get 1/2 can (small) of Sheba or Wellness in the morning, and 1 can (small) each of Sheba or Wellness in the evening. They also get greenies (though only one of them wants it) as snackies during the day. One came to use with food obsession issues, because he was a street kitty. We've only had them for a month now, and they've each put on a few ounces, but are holding steady at around 10 lbs, which is a good weight for both of them.