Within the fitness industry, a thin fitness tracker is often desired over a bulky one. The thinnest fitness tracker on the market is the Fitbit Flex 2. This fitness tracker is mostly a heart rate monitor, but its automatic all-day activity and sleep tracking also shows you how your entire day adds up, and the handy SmartTrack automatic exercise recognition records your exercises for you.  This fitness tracker/heart rate monitor measures 2.37 x 8.75 x 2.37 inches and 5.61 ounces. As mentioned, this is mostly a heart rate monitor instead of a fitness tracker, but the ultra-thin design is perfect for those with thin wrists.