Here's why:
* Comparative adjectives show a difference between two things. They often end in "-er" (e.g., sadder, cleverer, fatter) or use the word "more" (e.g., more sad, more clever, more fat).
* Least is a superlative adjective, indicating the smallest degree of something.
* Fattest is a superlative adjective, indicating the highest degree of something.
Let me know if you have any other questions!