It's about the balance. Kevin Anderson has better balance which helps him serve great. At his trophy position (kzfaq.info/get/bejne/m96Te9iX2rO5XZc.html), it looks quite similar to Hyeon Chung's trophy position (kzfaq.info/get/bejne/f5OXqJVj3ae9cmw.html), except their right foot. If you pause at those moments and take a look, Anderson's right foot is more open, which he can use to support his upper body, hence he can balance better, load his body slightly backwards before release the energy. Anderson doesn't look like sitting that much :-D. Chung's right foot is more closed (almost parallel to his left foot). Hence he can't load his body back, or he'll lose balance (you can test yourself :-D). We can see that he mostly put his weight to his left foot, not between the feet. He doesn't take advantage of his right foot to get more energy. Gäel Monfils has similar feet position to Hyeon Chung (kzfaq.info/get/bejne/mJtolcZlm93YlIE.html). However, Monfils doesn't try to push his hips forward as much as Chung, but keep his weight nicely between his feet. Monfils has great balance and good serves. Talking about the feet position, Andy Murray has similar pattern to Hyeon Chung. I believe it prevents him from having good body push for his 2nd / kick serve (kzfaq.info/get/bejne/pLSdn7aGqtqZlIU.html - This moment is his 2nd serve). In the kick serve, the body needs to moving up together with the swing to provide more energy. Murray hardly can do that. He has good balance for his 1st / flat serve though. I have a video about body movement that may relate to Andy Murray's serve: kzfaq.info/get/bejne/jMp9ZZOhtMyaYKc.html.

@@TenniswithAlex kevin Anderson is 6ft 8 and chung is 6ft 2. That height difference makes a huge difference to leverage and creating a legal serve that goes in & fast. Kevin Anderson could serve fast even without his legs due to his height.