I don't care about the size too much, but small has the advantage of one-handed use, of fitting better in my pocket and being less likely to break.

I kind of care about the design. It's elegant-- I like the flat camera, the chamfered edges and the brick-y form (which I appreciate they brought back with the new iPad Pro).

What I care most about though is the headphone jack. This is why I still keep mine around in spite of having another phone-- to listen to music.

EDIT: The fact that it's cheaper, so I could get more memory for the same price as a bigger model also contributes to it still sticking around primarily for music, but also with some games.