As Substance said, Importing Stuff is the #1 File size increase of the whole thing.
Next is Music <Imported>
GUI Triggers
JASS Triggers
Terrain
Custom Spells
Other-Placed Units (Loading Time)
This is what I personally think, I could/probobly am wrong.
Music is definitely the top spot on the list of stuff that blows up map size like there's no tomorrow.
Followed by skins, or other graphics like loading screen.
Followed by models.
Followed by smaller imports like icons.
Triggers or custom objects / spells?
Nah... even 500 triggers is nothing to worry about.
As for terrain... well, epic maps need epic size.
Still, 1. you do not really need an epic map, 2. decent terrain is of major importance.
Never worry that you might place one tree too many. Do worry that you didn't place one enough.
Of course, once you hit the destructible / doodad limit, you should stop. Preferably sooner.
Anyway, just forget music. You're the only one anyway with that weird taste...
Finally finding about some of the bots that are flooding the users online - bytespider apparently is a huge offender here - ignores robots.txt and comes in from a ton of different IPs
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.