I can’t be overfeeding because my dog only eats a bit and leaves the rest.

If your dog picks at his food and leaves food behind, this means he is eating as much as he wants.  This is more than likely more than he actually needs.  He could probably manage with a little less on a daily basis.  His health would improve and he would probably enjoy his food more if he ate less.

But my dog isn’t overweight.

Many dogs which are overfed do not put on weight but have itchy skin, bad breath and fur loss.  This tends to be true for dogs who have lots of exercise.  The dogs who are exercised less will put on weight.

…  Anne