As I'm a type 1 diabetic I don't really want to start giving advice on type2 diets.
What I will say is that your diet will reflect your body weight. Your current diet may be excellent and your body weight will reflect that, whether it means gaining or losing weight shouldn't be to concerning for you.
To increase your body weight with muscle mass would ultimately require a change to your diet I suspect, along with a training/ exercise regime.
Sorry can't really advise anymore than that,as my diet, for my lifestyle probably wouldn't suit you as a type 2 diabetic.