Apple's been continually pushing this issue. The original iPhone had a mini SIM, which is the one we generally think of as the standard size. The iPhone 4 moved to a micro SIM, which is the next smaller one depicted in your picture, and the 4S shared that. The 5 then went for the nano SIM. Presumably they're short enough on internal space that shrinking the SIM card actually makes some noticeable difference to them. The downside is that us users get to play games with getting (or cutting to size!) the right kind of SIM card for our particular model of phone.
I recall talk of purely internal, reprogrammable "SIM cards", so maybe that's the next step for Apple.
