ASUS U2E: Form vs. Function
 

In the quest to reduce component footprints, companies have created smaller standards over time, one of the latest being 1.8" hard drives. The problem with such a small physical hard drive size is twofold: storage capacity is obviously lower, but performance suffers as well. Finally, tying up all of the above points is the topic of battery life. What's the point of making a small notebook if you still have to lug around a large battery in order to get reasonable battery life? Thus, performance is often further reduced in order to keep power requirements in check.

http://www.anandtech.com/mobile/showdoc.aspx?i=3286