If it turns out your dog is underweight and you feed them more than enough, you might need a visit to the vet to make sure there isn’t a malabsorption issue or other ailment causing it. Unfortunately, a lot of dog foods (even the better and more expensive ones) can have synthetic vitamins and minerals added. As well as mineral supplements you might be purchasing to add to a homemade food, so make sure you read my article about Ingredient Education to check yours.
So what does this mean? It means that anything not coming from a natural, whole, source is going to be more difficult for the body to absorb and digest. For example, think about when you take a vitamin, most of it definitely goes out the other end on the next bathroom break, BUT if you were to drink a fresh juice, filled with tons of vitamins and minerals, less gets lost to the restroom. It’s the same for dogs. Dogs who suffer from malabsorption can be notorious eaters of poop, dirt, grass, lick wood, sheetrock and a number of other things. The instinctual nature that they have is to find the minerals they lack. While you’re thinking the dog has just lost their dam mind, they’re actually giving you clues to what is going on inside. IF this is the case (and I am by no means telling you to self diagnose your pet, a holistic vet visit is always recommended), it is always best to feed them a fresh, whole food diet, that is gently cooked with a mineral supplement derived of natural sources (honestly my advice for ALL dogs). My last Boston Terrier Trixie suffered from this and I developed a lot of my products to solve the ailments she had.
It’s also possible your dog has a food allergy. Allergies can cause a digestive system to rid itself of food immediately, since it is actively aggravating the system. it comes out in a fast, and negative way (aka diarrhea) on the other end. It’s best to review your food AND treat ingredients for any No No’s on my ingredient list.
Lastly, your dog might just have a really really high metabolism, let’s call it the model figure, #blessed. I’ve had dogs who ate twice as much as another dog twice their size and were at the perfect weight, and that’s okay! Just like humans, all dogs are different, this is about knowing your dog so you can be their advocate for living the longest, healthiest life possible. For this case, I recommend making sure they’re on a high protein diet (60% protein min, higher than that won’t make a difference), this will keep weight on them and make sure they aren’t actually starving the whole time!
It's also important to note that dogs will lean towards the thinner side in the beginning and end of their lives. Puppies can grow so quickly that they enter some bony, lanky stages. And geriatric dogs can also lean towards the skinnier side as they get closer to their departure. It is very important at both these stages that you're getting regular visits in to the vet to assure this is the cause. Especially in the end stages of life, there can be a slew of causes that can be treatable.
Being educated and aware is your best bet as a dog parent, so make sure to also read my articles under the nutrition section if your dog is suffering from being underweight!