A lot of Greek words, like ταχυδρομικός, are compound words. Usually, there is the word root, which may or may not be a compound, and the ending which specifies what type of word it is (e.g. a verb, a noun indicating location/gender/other meanings, plus conjugation/declension forms etc.). Ταχυδρομικός is formed of ταχύς (fast) + δρόμος (road) + -ικός (adjective ending). Ταχυδρομείο (post office) has the -είο ending signifying location. Sometimes there are prefixes too.
You can read more on how these grammatical phenomena affect word length here: https://en.wikipedia.org/wiki/Word_formation.