I agree, it does seem weird, but...
The ZP120 has about 2.5 times the volume of the ZP90 and has ventilation holes top and bottom and a cast aluminium casing - as opposed to the polycarbonate casing of the ZP90 - and, as Sonos say themselves: "Aluminum casing also facilitates passive cooling." So, I think the ZP120 probably does produce more heat than the ZP90, but it dissipates it better, so overall the unit remains cooler. With the ZP90, whatever heat is generated inside the unit gets trapped in side, as there are holes at the bottom, but heat rises and polycarbonate is pretty useless for heat dissipation...