The LGD owners need to holler here, cause I don't have one, and they may be different but...
The only feedback that I can give you is what three vets told me , about Merlin, my dane. For his first two years, I fed that dog non-stop, quality dry food, canned food and table scraps, and you could see every rib and count the knobs on his spine. I would take him out for walks, and people would threaten to call the humane society because I was starving him. I spent more on food for that dog than I spent grocery shopping for the four humans in the house! I took him to three different vets, had all kinds of tests run, and they all said I was obsessing. His coat was shiney, eyes bright, and musculature properly developing, so the only problem was mine. If a dog was actually starving it would affect growth, but as long as they were getting proper nutrition and the other factors were fine, then that was how he was meant to be. According to all three, the size of an animal is decided genetically, and only affected by environment in extreme cases of neglect and abuse.
If it makes you feel better, add a bit of soft food to every meal, keeping in mind that you need to keep her on the slim side for health reasons, but don't do like I did and spend hundereds of dollars only to find out that there is no problem with your dog. You can see that Merlin is a strapping fine healthy (still thin) 4 year old.
Hope that this can at least ease your mind
Stella