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Work in progress so let me finish this before we get into technical questions. Thanks.

This is less about making you second guess and more about dealing with the rare moments of why what is obvious motherboard got peripheral (or the dreaded midgrade).
Thing is in reality 9 or more out of ten, a motherboard is a motherboard.
The rest? And here's how these have broken down for me.

Has a big socket or has a MASSIVE CPU. But it's small socket. Why?
Let's start with small socket motherboards that have large, over 2 inch sockets.
Win-CPU.
Via
Grand
Gigabyte
Packard
AT&T
These are from the late 70s through early 90s. Though by modern terminology these were computers in reality they were still terminals. They were wholly dependent on being connected to a server, mini computer or mainframe.
Today you can load and run many flavours of Linux and make these usable computers but dos and CPM will choke.
These systems do the display and basic software loading but anything more complicated than make this pixel this colour is actually done by the connected host. The usually have an I/O controller and memory controllers and a host of rom banks. A single CPU and a bunch of rom DIP sockets.
On the largest most expensive boards if you leave ALL the rom sockets filled you may be able to get telco. On the other hand if all the rom and ram banks are empty or less than half the board is dip sockets; were at small socket or midgrade.

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Next we have the huge CPU soldered to the board. These are 386/486/586 era computers. To pun Star Wars it's the revenge of the clones.
The brand new Apple and IBM computers were $2000-$10000 but these clones were almost as good, almost the same; and cost $900 or less! Some via based computers were under $300!
Basically the put just enough in the design. Then "permanently" soldered everything to the board. They were designed to do the most computation at the lowest price. We're looking at Tandy diamond, leading edge, star etc.
First there's a fraction of the precious metals content because there's few or no sockets.
Second the components are incomplete and/or cast off equipment. CPUs with missing registers and cache. Ram with missing banks. Little or no floating point computation. When IBM had 50 million damaged ram chips in 1987 the sold them to Tc limited. Who put them in these supply chains for a fraction of normal cost.
Finally any damage to these boards makes recycling that much more difficult.
These boards were notorious for burn outs. And a burnt board, much like burnt wire, is much harder to properly recycle. Making the overall value lower.

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Giant soldered CPUs.
I have an at or xt board and the CPU is soldered on. It's 8 inches wide. Why is this a small socket board.
The multi chip carrier.
Today every mainstream CPU is a mcc. They have 2-16 CPUs, the math coprocessor, and multiple caches on a single "chip".
All in under a square inch.
Back in the 70s and 80s the push for more speed, less delay, lower latency and lower prices lead to the development of the mcc. The multi chip carrier put all the essential chips and components on a single pcb. Dropped the pcb on a ceramic base. Then dropped a ceramic cover on top and epoxied it closed. Then soldered it to a board . These have much less tracing on the motherboard. They have fewer components over all. The boards are worth a fraction of the contemporary equivalents but made up for it in the scrap value of the mcc. These boards are rarely over six inches to 12 inches and the mcc is 90%+ the board's population.
IBM is the worst offender in the mcc design world with units exceeding 8-10 inches square but most were between 3 and 5 inches.

If you remove the mcc it will generally go as pinless CPU but the board will then be low or mid grade.

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Micro boards.
To date Micro boards that I've seen have always fallen into two categories and neither is small socket.
A Micro board is a smaller than standard or non standard small board of custom design. The have little or more often no expansion. These are set top computers e.g. Cable boxes, book shelf computers, e.g. View and netbox, and the vast majority of video game boards. All of these falling into the peripheral board category

The flip side to this is extremely high end av equipment and home theatre computers. Most of which use gold flashed gold sandwich boards with massive amounts of ram multiple DSPs and major high grade CPUs and controllers. These few normally go as telco.

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