The G4 tower is known for being quite noisy for a Mac.
The Mac Mini and iMac are supposed to be very quiet. Tom Hardware tested the Mac Mini and found "During testing, our Mac mini was able to shine in this respect, drawing a mere 20 watts of power; during DVD playback, this rose to only 28 W. In contrast, the power requirements of current Intel-based PC systems is anything but reasonable - under comparable conditions, these power-hungry machines draw up to 160 W". That means much less heat to dissipate. The power supply is external which also cuts down on heat in the box.