openSPOT2's Wi-Fi transmit power is only 15 and 14 dBm (depending on the link speed, please see the specifications at
http://manuals.sharkrf.com/openspot2/en/specifications.html
), and the built-in Wi-Fi antenna is optimized for short range use and therefore it is small. Other devices like mobile phones, laptops, tablets use much larger antennas and far more transmit power.