In my short experience I have found out that to win you need to have your opponent fold when he has the best hand, else if you just go to showdown every hand, best cards win and you cant regularly win.
The only way to acheive this is via intimidation and to intimidate you must be aggressive, the player that only calls will never make anyone fold right ? Some ppl are tight agg and are feared as they have the goods when they come in but the loose agg are as feared if not more, I noticed that as soon as they have a reasonable stack players avoid them at all cost and hope for other players to take them down, or only come in with the nuts.
You also mentioned that it can be a disadvantage to be aggro with short stacks, well I am always seeking to eliminate players personally and I will take extra risks to do so. Not just to increase my chances to win by having less competitors but due to the added fear and respect I get from the other remaining players particularly at 4-5 players when so many players just passively wait for the smaller stacks to be eliminated.
I guess it is what every book and everyone says, to win regularly you must be aggressive, the real hard part is to learn when to back off
