why the fat stylus?

The iPhone doesn't have a stylus - you use your finger. Are you asking why your fingers are fat?

Because the iPhone and many other smart phones use a capacitive sensing layer. Older PDA style handheld units used a pressure sensitive resistive matrix.

The resistive type of old will sense any pressure applied to the screen there by allowing for a pencil point type of plastic stylus.

The capacitive sensor used in most modern touch screens use a capacitive method of sensing a screen touch. So any stylus designed for a capacitive touch screen must be "massive" (Fat) enough and of a material that will influence the capacitive sensors adequately. To test this try touching you iPhone screen with your fingernail or one of the old style nylon (pencil point) styli, you will find neither will be seen as a "Touch" by your iPhone.

That said, I don't know how the new Samsung "Note"smart phone works because it does use a stylus that looks link one of the older pencil point nylon styli. It may use a combination of both capacitive and resistive matrix sensors built into the screen. This is something that you have now perked my curiosity about and will probably motivate me to do some Googling for an answer. I don't know if there is some magic incorporated into the "Samsung Note" stylus but now that you have peeked my curiosity I may go by the the ATT store today and try the "Samsung Note" stylus on my iPhone 4S. I don't expect it to work BUT if it does I will post back with my findings, right after I purchase one to use with my iPhone. 😉

why would you bother with all that? use your finger....thats what iphone are for.