Hi Gabrielle, welcome!
Congratulations on your new puppy!
I don't have long term comparisons to make, but it has been my experience, as a breeder, keeping litters until around 10 weeks of age that the runts generally catch up with the others and sometimes out grow them!
In fact, unless there is a health issue with the runt, I don't see any reason that it should not thrive the same as the others.
There are some puppies called runts because they are smaller and often those pups ARE genuinely smaller than the others but I believe that in those instances it is because the pup is simply taking after one of the parents or one of the dogs in the pedigree which was smaller in size. Still they are very healthy and strong.
It sounds like your baby is catching up and may be the same size as the rest of the pups in her litter.
We'd love to see pictures and hear more about her!