Thank you for responding! I'm SO glad to hear that she has had her shots. <whew!> As a purebred Chocolate Lab, even as a runt, she should be 3-4 times her present size. If you have really been feeding her 1 cup (like as in 1 cup measured) of food per day, she has a LOT of catching up to do and it's NOT too late for a more or less "full recovery" and growth to at least close to proper size.
3/4 of a cup of food is NOT enough... not even if fed 3 time a day. You must have misread my earlier post... What you are saying is you will be increasing her intake by 1 & 1//4 cup per day to 2 1/4 cups total.
That is NOT enough food! That is ONE HALF what she should be getting.
I am NOT trying to be mean, and there's no real way to say this in a nice way. You are starving that puppy.
She is NOT 13 pounds because she was the "runt of the litter" She is 13 pounds, because you are feeding her like she's a chihuahua puppy.
Based on your food choice, right now, you should be feeding her 2 cups, THREE times a day! If you upgrade to a better quality of puppy food for her, you could probably drop to 2 1/2 cups twice a day.
You are feeding her Purina Puppy Chow... I'm sorry, but that is actually a very poor nutrition food. it is mostly grain, garbage, and chemicals. Here are the ingredients. I highlighted in red the items that are NOT chemicals: "
Ingredients Whole grain corn, corn gluten meal, chicken by-product meal, animal fat preserved with mixed-tocopherols (form of Vitamin E), soybean meal, egg and chicken flavor, brewers rice, barley, animal digest, calcium phosphate, fish oil, calcium carbonate, dried yeast, salt, potassium chloride, choline chloride, L-Lysine monohydrochloride, L-ascorbyl-2-polyphosphate (source of Vitamin C), zinc sulfate, Vitamin E supplement, ferrous sulfate, Yellow 6, DL-Methionine, Yellow 5, manganese sulfate, niacin, Red 40, Vitamin A supplement, Blue 2, copper sulfate, calcium pantothenate, garlic oil, pyridoxine hydrochloride, Vitamin B-12 supplement, thiamine mononitrate, Vitamin D-3 supplement, riboflavin supplement, calcium iodate, menadione sodium bisulfite complex (source of Vitamin K activity), folic acid, biotin, sodium selenite"
https://www.petco.com/shop/en/petcostore/product/purina-puppy-chow-complete-nutrition-formula It's "flavored" with chicken and egg "flavor" so the pup will even eat it.
Here's some info for your reading pleasure:
https://www.dogfoodadvisor.com/best-dog-foods/
https://mysweetpuppy.net/best-large-breed-puppy-foods/
http://bigdogshugepaws.com/nutrition