Power supplies have been down-sizing for years for various reasons. One is that transformers have gotten smaller and lighter using newer core materials; some designs may utilize switching supplies that are very lightweight. More likely is the recognition that peaks are not sustained for long, so a smaller core is acceptable since it need not provide full power on a continuous basis. Smaller transformer core, smaller filter capacitors, less heat sinking, less weight... I am sure there are other reasons I haven't thought of in the past minute or two. In the real world, as has been implied/mentioned before, for most people (speakers, rooms, volume, etc.) it does not matter. The AVR can supply more than enough power (few Watts) during normal listening and has the capacity to provide brief peaks as needed.
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley