Is it normal for two 6 month old kittens to eat only one can of cat food all day?

I have to 6 month old kittens and apparently one 5.5 oz can of Blue Buffalo Spa Select cat food is enough for them. Is this not enough? How can I get them to eat more? They only eat a little bit of it and then I refrigerate it, they eventually finish the whole can at the end of the day.