i won't say the startup are less ambitious because i think almost all of them wants to reach the level of success of google/facebook , but then it is all about your idea/product, team, execution, timing, funding, luck, mentoring, etc.

Also YCombinator is a king-maker but not every king can have huge empire and not everyone can become Alexander, though everyone would like to become one, but then they all have some kind of limitations which is sometimes is out of your control.