What do you call a person who dreams big, but then does much bigger? In a good way, that is.
- one who achieves success over and above the standard or expected level especially at an early age
If you're looking for something that better captures the "dreams big" part, you might also try:
- a wonder child or child prodigy.
- a person who succeeds, especially in business, at a comparatively early age.
Perhaps some combination of the two?