Cut back, attenuate, chiffony, angular, inadequate, dilute, feeble, faint, become less dense/numerous, and become less in number are all synonyms for thin. Thick and broad are these antonyms of thin. And here are another 123 synonyms for thin.
1. narrow
2. lightweight
3. slim
4. sparse
5. watery
6. insubstantial
7. weak
8. decrease