Could be in her genes.
maybe all battle-oriented Belkan rulers can turn into adults when they go to battle when they're only four years old and turn back to normal when they're at home?
Plus, Saint Kaiser Vivio was at least as powerful as the S+ Nanoha, most likely stronger, just lacking experiences when they fought.
Loli!Vivio in StrikerS doesn't really rate on the scale, remember Schach telling (Signum or Nanoha, I forget who exactly) that her body was normal and her magic wasn't any stronger than it should have been for her age.
Vivid!Vivio is A-rank (according to the test paper) in magic, but as an Adult, it's safe to assume she's closer to AAA or even S rank, since people clear the way for her and Nove to spar.