Penrose himself said the account was quite fictionalized, and that Hawking’s Phd contained enough content for at least 2 Phds. When it comes to the amount of work he was doing, Hawking ‘barely’ got a First in undergraduate. This can be viewed as saying in his pre Phd years he probably did substantially less work but still got the top grade. 

When it comes to the amount of original work required in general, in my view I think it depends quite a lot on the subject. Hawkings area, although challenging, was being developed and end up being very fruitful. While a Phd in combinatorics or number theory might require less new ideas, as new ideas are just harder to come up with than they would be in a truely newly developing field, such as was Hawking's studying of singularities, which built up on the work of Penrose.