The KAT system has a suite of different devices.
The original KAT device  is compact (2 in x 2 in x 1/2in ) and can easily be attached to the wearer’s body,  tucked into waistband, sock, tight clothing or sport equipment.
Why aren't they smaller? 
Because KAT is NOT primarily a data-collector;  it is an Active Training and Feedback device.  Vibrations are felt on the body through the surface area. This  2 x 2 surface area size allows for an optimal  balance of  'felt sensation' and portability of the system.