First, he likely got that wealthy by taking aggressive positions. Meaning he is comfortable with it.
Second (and this is obviously an extreme example). Say I have $50m. I borrow another $25m. I put all of that money in a single concentrated position, that is very likely to go up (i.e. Amazon). It doubles in a year. Now you made $75m (excluding some interest payments).
First, he likely got that wealthy by taking aggressive positions. Meaning he is comfortable with it.
Second (and this is obviously an extreme example). Say I have $50m. I borrow another $25m. I put all of that money in a single concentrated position, that is very likely to go up (i.e. Amazon). It doubles in a year. Now you made $75m (excluding some interest payments).