I have a 4 1/2 month old female she is 42 lbs. I was worried she wasn't that big also, but the more I look she is just fine. She is filling out nicely and is happy. She eats plenty and gets tons of exercise she is all muscle we walk her 3 miles per day and she needs it. I agree if you are concerned check around and check with your vet.l
How much does your dog weigh? I have a male that is 4 1/2 months he is only 41 pounds. I was told by a breeder that this breed gets their size in the first year and their weight in their second year. If your vet isn't worried I wouldn't be either.
Right now what you are probably seeing is her leggy stage were it seems like the only thing growing is her legs. Dont worry they all go through it. my first AB didnt start to fill out until he was about 8 months.