
If that was my breed I wouldn't think the pup underweight, but have no idea what size your pup is. Can you see all her ribs and backbone? You should be able to fel them easily under a light covering of flesh and just feel the backbone, but it whouldn't be prominent. The back ribs should be easy to se and feel,a nd there should be a definite waist.
As with people one person will weigh more than another and both be the correct weight for their build and height.
As rtegards additives to the food. Think about it If I gave you something scrummy like gateux (if you like cakes :D) or a second helping of meat and veg I would go mad fthe cake, doesn't mean I need it :D .