you are running linux arduino and wifi.
Wifi is know to use energy.
If we say 0.3 amp 5Volt that makes 1.5 watt, right?
Seems pretty acceptable to me if you use all of its features 
I know you won't run for a month on a battery with this power consumption but I feel it is acceptable for running a wifi connected pc with arduino.
I noticed that yun is getting warmer than standard arduino's. I'm not saying it is hot but it is warm after running for a while.
Best regards
Jantje