Past winners have hit it big because they received a lot of exposure in a short amount of time. As a result the winners are more likely to get deals with record companies. They are also more likely to be searched for on Google and YouTube and gain exen more exposure.
The winners of America's Got Talent receive the chance to perform their own show, in front of important audiences, a very important step in hitting it big. They also have received lots of publicity from merely participating, and doing so well in, that many stages of such a large TV show. In these ways, they have the set up to hit it big, as the winners of America's Got Talent.
Obviously not all contestants on America's Got Talent make it big, but undoubtedly many do. Even the ones who are talented but not enough to win get the exposure they may need to be noticed by talent scouts or even by the public who may demand to see more of them. America;s got talent does indeed create stars!
I can't recall anyone who has really hit it big from America's Got Talent. I am sure there are a few who it has opened opportunities up for but don't feel like it is anywhere near American Idol past winners. I just don't think the show holds that much influence.