Sometimes. Short words can have long entries because they have a wide variety of uses (think of "get" or "go"), but not all long words have short entries.
Also depends on the dictionary the word is in. For our Webster's Third New International, Unabridged, there were some insanely long definitions for words like "oxygen" and "hotel" and "heart"--not because the words have a wide variety of uses and therefore have complex meanings, but because it was an unabridged dictionary, and people expect an inordinate amount of information in them.
Here's the now infamous definition for "hotel" from the Third:
a building of many rooms chiefly for overnight accommodation of transients and several floors served by elevators, usually with a large open street-level lobby containing easy chairs, with a variety of compartments for eating, drinking, dancing, exhibitions, and group meetings (as of salesmen or convention attendants), with shops having both inside and street-side entrances and offering for sale items (as clothes, gifts, candy, theater tickets, travel tickets) of particular interest to a traveler, or providing personal services (as hairdressing, shoe shining), and with telephone booths, writing tables and washrooms freely available