While size is the main focus, there are also a few other improvements being toted in the Sprint -- it sports a micro-USB port for charging (changed from mini-USB), a softer cable between the buds, an inconspicuous notification LED, enhanced call quality and the multifunction button has been repositioned to the top of one of the earpieces (the JF3's is on the outside).
ENGADGET: JayBird intros Freedom Sprint: the JF3 Bluetooth stereo fitness headset gets 40% smaller