Here's why:
* Expressive is an adjective.
* Comparative adjectives are used to compare two things.
* To form the comparative of most adjectives, we add "-er" to the end of the word.
* However, "expressive" is a three-syllable word, and for words with three or more syllables, we use "more" before the adjective.
Therefore, the correct comparative form of "expressive" is more expressive.