Each sound is a syllable.
Turkey is two sounds: Tur and Key. Syllable is three syllables: Syl and Uh and Bull. Syl-Uh-Bull.
Words of one syllable (big, fat, tall) usually form their comparative and superlatives by adding -er and -est (bigger, biggest, fatter, fattest, taller, tallest). Notice how the single final consonant is doubled, in order to keep the short sound of the vowel. If we didn't double the t in fat (fatter) it would be spelled fater sound like "fate-er."
Words of three or more syllables (grandiose, incredible, wonderful) always form their comparatives with more or most (more grandiose, most grandiose, etc.)
Words of two syllables (pretty, happy, simple) are usually optional. More simple = simpler; most pretty = prettiest. Either works.
There are exceptions. Some one-syllable words (huge comes to mind) can't be huger or hugest; only more huge and most huge will work.
And some two-syllable words (graceful comes to mind here) are not optional. Only more graceful, not gracefuller, is accepted.