Questions & Answers about No quiero el pequeño; prefiero el grande.
Because in this sentence they are being used like nouns: el pequeño = the small one and el grande = the big one.
Spanish often uses definite article + adjective to avoid repeating a noun that is already understood from context. For example, if you are choosing between two shirts, el pequeño means the small one.
Spanish very naturally leaves out a noun when it is obvious from context. Instead of repeating something like el vaso pequeño and el vaso grande, a speaker can simply say el pequeño and el grande.
English does this too with the small one and the big one, but Spanish uses the article plus the adjective directly.