https://www.bluetooth.org/foundry/si..._presentations is a useful link on the Bluetooth special interest group's website.
One other factor is that both Bluetooth and WiFi frequency hop (unlike basic GSM / GPRS), ensuring they are tolerant to interference, and also interfere less on any particular frequency within the operating band.
Bluetooth has three power classes 1mW, 2.5mW and 100mW, giving nominal ranges of 10m, 20m and 100m (although three times that can be achieved with good receivers). Compare that to GSM which potentially can be used to 35Km, or even 120Km on extended range, take into account the inverse square law for propagation, and you get some idea of the power comparisons.
Hope this helps.