Here are the corrected forms for each sentence based on the structure of comparative forms:

11. The apartment is big. The rent is high.
-> The bigger the apartment, the higher the rent.


12. We set off soon. We will arrive soon.
-> The sooner we set off, the sooner we will arrive.


13. The joke is good. The laughter is loud.
-> The better the joke, the louder the laughter.


14. She gets fat. She feels tired.
-> The fatter she gets, the more tired she feels.


15. As he gets older, he wants to travel less.
-> The older he gets, the less he wants to travel.


16. The children are excited with the difficult games.
-> The more difficult the games, the more excited the children are.


17. People dive fast. Many accidents happen.
-> The faster people dive, the more accidents happen.


18. I meet him much. I hate him much.
-> The more I meet him, the more I hate him.


19. My boss works better when he is pressed for time.
-> The less time my boss has, the better he works.