Nope no thyristors in these. Just checked. Just a variable autotransformer. (Not a resistor or rheostat).

The ones installed in a standard workbox are not purely a rheostat/resistor/autotransformer. Older dimmers that don't involve a chopper circuit are much bigger, but certainly do exist. It's possible that a 100-150W unit could be fit into 2 gang box, but I've never personally seen one.

Variable autotransformers do not need such close load matching and definitely have been used in residential applications. Bigger than your standard wall switch, but sold in a form factor to be installed in a standard 2x4 studwall

Cast aluminum boxes actually. Luxtrol WBD series.

That is a dimmer. It says right on the cover dimmer. It's a variable autotransformer in a box,.which will dim incandescent bulbs just fine. But maybe only a hundred or so watts looking at it's size.

I have in my possession several Luxtrol WBD series lighting dimmers with a similar electrical configuration that were removed during a dining room remodel.

I have some Luxtrol WBD series variable autotransformers that were removed from residential dining room applications. They are much bigger than your standard wall switch, but designed to fit into a wall cavity.

Nope. Definitely a thing. Also I'm corroborating your claims that they are too big to fit in a standard electrical box.

I just pulled mine out of basement storage and took a look. Inside is a variable auto transformer. Lookup "Luxtrol WBD" on eBay to get an idea.

I've seen them in homes, typically dining room chandeliers. They are big and got hot, just like you state.

I have some actual rheostat dimmers I removed from an old dining room. They weigh about 10lbs each and are in cast iron boxes that are about 5x4x3 inches in size.