[mikepurvis]

Some of the bigness is just tradition and buyer expectation (big = expensive). But also, modern AVRs are like 1000W devices amplifying 7, 9, even 11 channels of passives. That’s a lot of componentry and corresponding heat to shed— if you open one of those up, it’s not just empty space in there like an NES cartridge or something.

[recursive]

That makes some sense, but for those of us with two channels maxing out at 25W each, there seems to be some use for a smaller one. I think there are more people on the small end of the spectrum than those with a big surround deployment. I suppose they're mostly using sound bars with an HDMI input.

[mikepurvis]

... that said, there is also a small market for "separates" where you have a decoding-only preamp that either feeds active speakers or another box containing just the multi-channel amplification:

https://www.marantz.com/en-ca/category/av-separates/

The output of these units is line-level signals feeding high-impedance loads. They could definitely be a fraction of the size they are.