Mid-stack tight players get as tight as possible. They have too many chips to shove, and by definition they don't often call or 3-bet preflop. Therefore, all they have to do is throw away many hands that they would have played under different circumstances.
Sitting directly to your left is a tight middle-stacked player - it's like being on the button twice a round. This is the ideal situation. For these reasons, I sometimes even call shoves from the tight short stack to my left. If I call and lose, that player will double up to mid-stack and play even tighter, allowing me to steal the blinds from him more often.
Loose players usually don't get tighter with the average stack. Some will play as if they are short stacked and will play lightweight with the big blinds than most other players. Others will play like they have a big stack and 3-bet lightly or speculatively call.
None of these situations work for me if I'm using pure style, so when I'm out of position with these players, I'll open the game with a tighter and more playable range. That way, if they choose to shove back, I can call them more often, and if they choose to loosely call preflop, then I can play the stack more often postflop. Worst-case scenario - if you open a wide hand, throw a marginal hand and lose the pot to them - then you end up with a loose aggressive player with a large stack to your left. This must be avoided
__________________
|