Actually the secret to prevent leakage around the thigh is not looking at the number, but adjusting it to be more fitted at the thigh region, especially for brands like Merries which have bigger diapers for the same size. Huggies Gold/Drypers/Mamy Poko Air Fit are more snug, I find. You can still prevent leaks with Merries, but adjust the diaper to make sure the thigh area is very snug.
Weight aside, you should also gauge the size of her thighs and if the NB diapers are causing red marks on her legs/thighs (signs of tight diapers)