Depends on the drive really. For example the 80gb ipods drive vs the 160gb ipod...they have different number of platters.
EDIT: For example read the third paragraph of this: http://news.zdnet.co.uk/hardware/0,1000000091,39263866,00.htm
Dude, you should check the date on that, it's posted 2006! Well outta date! I'm pretty sure technology have move on since then... No fool gonna be installing 1.8" drives or even compact 2.5" laptop drives in a desktop machine... will they? Anyway, I was thinking and comparing the reliability issues of workstation and server drives, where performance matters...