This thread is reassuring to me, since I have the same concern, but since there are several cases like us that later reach the standard then we have to just forget about scales and concentrate in that our pups are otherwise healthy.
Diabla is 15 kg (about 33 pounds) at 5,5 months old, with 51 cms, at the shoulders (20,07 in.) and the standard says 55-60 cms. so she's not so below it. Both fathers are within standard, being the mother even in the big side of it. Which is the height of Mace? I tend to thrust more in that than in weight with fat dogs as much into fashion than even vets tend to forget what a lean dog is.
She must have gone through a sprout this last week, since she got suddenly a bit skinny for my taste so I hope to put at least 1 more kg. on her in the next days adjusting his ration and adding some eggs. She used to have lose stools but I finally found a food that suits her and now they couldn't be firmer. We had a vet appointment last week and she is in perfect health.
At the age of your pup she got into her food bag (again) and you should have seen her belly... she looked like a donkey