[foldr]

I think that's actually an interesting example of how C sees itself as a high level language (from the perspective of the early 70s), even though we now categorize it as low-level. You'd expect the default integer type in a high level language to be able to store values that are large enough for an interesting range of practical tasks; 16 bits is arguably the bare minimum to meet that requirement.

[mananaysiempre]

IIRC a retrospective by one of the designers listed the default promotions as a careless mistake, because the original—offhand—reasoning was that the registers are 16-bit anyway, so they might as well widen everything when it’s free (on the only machine the language was implemented on at that moment).