If you are the chip leader, you should be concentrating on stealing the blinds or buy the pot with mediocre to crap cards (but not all the time). The short stack is indeed the niggest threat because they are playing sper tight and only play great
hands so unless you have a good hand, don't bother to call. Medium stack is not quite in the same position but they do play cautiously. Since they have more chips to work with, they might play OK hands. They could even steal the blinds at times. So, I think it might be wise to focus on the medium stacks. Be careful though! They'll take advantage of their position and once a premium hand comes up, it's all in.