Valkyrie cores respond to desires of improvement. In some cases the results are objective, but in others they are more subjective. I am going to pose a theory that the height and other appearance things are not some combat practicality deal, but rather a product of Anna's mind wanting to be like people she considered strong or adult. Physically being larger also being better is the sort of analogy that a young mind might make. Now, Anna may be shrinking out of desire to not stand out so much. She does not really seem to be interested in attention, or think that she is worth the attention that she gets, so wanting to look less unusual is possibly the reason for the ongoing change.