Puppy food SOMETIMES has more calories, more fat, more protein than adult. But you'll have to read the label. Also Lamb & Rice tends to be lower calorie/fat/protein. If you are feeding this Diamond Lamb Meal & Rice Formula for Adult Dogs - Diamond
it's 23/14 (protein/fat) and only 332 kcals per cup. NOT a food I would feed to a thin, hungry young dog.
I would not feed less than 26/16. But if you are going to stick with the Diamond line (not the Diamond Naturals), I would do premium adult or performance.
I would also aim for 3 cups of food a day and then gauge poops.
If it were me, I would get a back of Performance (or the Natural Performance) and mix it with the lamb and rice. Then keep her on Performance food.
Some labs are going to be rail thin depending on their pedigree. Mine isn't, she's a butterball on 3 cups.