Big Al was MUCH stronger as a highschooler than KO is now. He also has a very different style of low post game. Jefferson just has exceptional touch and footwork, that simply can't be taught, and while KO is above average in those areas, he is nowhere near Al.
Big Al was not that strong as a rookie. He was kind of fat, and not a whole lot of muscle as a rookie. It took him until his third season to get into NBA shape, because of his foot injuries his first two years. But he was 19 when he came into the league, and about 21 when he broke out with his 16/11 season. The big difference between them is really just their age difference. By this point Big Al was already a 20/10 guy, with great footwork and a nose for getting rebounds (although still slow-footed in other ways.)
I also think that KO is already a better defender than Big Al, simply because he puts effort into it. KO has all the footwork tools, and the exceptional touch that you mentioned that Big Al has; we saw that in summer league and preseason. What he lacks right now is confidence, and once that comes I think we will start seeing 15/8 games from him this season pretty routinely. And once the game starts to slow down for him he is going to be an important piece to this team offensively and pretty good defensively. The guy puts a lot of effort into his game and improving on both ends of the floor, from what I have read.