I think it's due to the nature of combat... Fate might wear things to minize painful bounce that might occur, while Nanoha being a sniper doesn't have to worry about such things
Ah... they both seem to have their adult figures (Fate not so much it seems, but it might be the vest doing it) yet their loli faces... a winning combo. But I was about to add that perhaps nature favored Nanoha bust wise. And Fate wearing such a vest would make sense.
