I don't think this is true. At best it's speculation. Apple hasn't released the power figures for the new iMac yet. But let's assume they're roughly 200W - which is consistent with the much more power hungry Intel iMacs. In reality, this iMac probably uses under 100W. There are power supplies capable of putting out 200W that are VERY small.
Here is one that is 300W and impressively small.
This company is putting out some very impressive designs too. The technology exists, they could have done it. They chose not to do it.