Aside from good quality food and proper exercise there’s nothing you can do to force her to grow bigger or heavier unless you-over feed her and turn her into a butter ball.
If it was a case of some magical feeding solution then every person on the planet would eat to make themselves taller, or shorter, stronger or weaker, smarter or not. But that’s just sci-fi and there’s no miracle growth pills, unless your starting with a hormone growth deficiency problem to begin with. At that time it would be medically treated.
Chances are your dogs genetics made her smaller and less stocky then some dogs of similar breeding. Even from a litter of 10 you could have 3 passive shy dogs, 2 monster dogs but not too bright, one superstar that’s intelligent and larger sized and a couple shy weaker pups. Even pure bloods have offspring... some will take from the parents, grandparents and great grandparents gene pool.