what Audyssey does, when you calibrate, is adjust the gain in all channels so the signal levels at the MLP is 75 dB for each channel individually. I've checked this after calibration with a precision SPL Meter, and the individual channels all play back at 75dB when using the AVR's test tones.
if your room is very large, then Audyssey will want more gain in all the channels so that the sound level at the MLP is 75dB SPL. it demands more power from the amps in order to get this level at the MLP compared to...
if you room is small, then Audyssey will want less gain in all channels so the signal level at the MLP is 75 dB SPL. it demand less power from the amps in order to get this level at the MLP.
Smaller space = less power for the same SPL.
this gain adjustment inside the AVR is like a preamp preset (for every channel) that supplements the master preamp volume control.
this has nothing to do with good or bad sound, only with individual channel levels.
in an uncalibrated system, there is no Reference level, but only Preference level. for someone with a good ear, the two will often coincide. it's more difficult to get right because one has to keep making volume adjusments until the right volume level has been found.
