As more people get involved in a fight the less individual skill is needed in order to win said fight. It's kind of like gear trumping skill in 1v1 fights, just because one side can muster more forces of lesser skilled people doesn't mean it should win against a smaller group of more skilled players.
Also, ambushing doesn't mean using stealth it just means attacking people where they don't expect to be attacked. In my experience smaller battles tend to be far more fun because you actually have a chance to win based on how skilled you are at the game. Large battles tend to just turn out to be large gank fests with a bunch of people killing one person and moving on.