Those are all true (though that an 8-bit microcontroller has 8-bit registers is… unsurprising). However I am referring to the instructions themselves, which we

Those are all true (though that an 8-bit microcontroller has 8-bit registers is… unsurprising). However I am referring to the instructions themselves, which were well-engineered to match the needs of a compiler. The result is very compact assembly for most C programs that you would write on an 8-bit microcontroller (including things like 16-bit arithmetic).