Originally Posted by http://wiki.guildwars.com/wiki/Loot_scaling
Loot scaling was introduced in the April 19, 2007 game update. The amount of drops you get are scaled to your party size and to the speed at which enemies are killed.
The following are affected by loot scaling:
* Common (white) items
* Gold
The following are exempt from loot scaling:
* Skill Tomes
* Scrolls
* Dye
* Rare materials, such as Ectoplasm
* Gemstones from the Domain of Anguish
* All other rare (gold) items
* All unique (green) items
* Special event items