As this paper [0] explains, the initial version of the C compiler for PDP-11 Unix was finished in 1972. And less than a year later (1973), people had ported the C compiler (but not Unix) to Honeywell 6000 series mainframes, and shortly thereafter to IBM 370 series mainframes as well. (Note the text of the paper says "IBM 310" in a couple of places – that's a typo/transcription error for "370".) Both were "larger machines" – the Honeywell 6000 had 36 bit integer arithmetic with 18 bit addressing; the IBM 370 had 32 bit integer arithmetic with 24 bit addressing.

Alan Snyder's 1974 masters thesis [1] describes the Honeywell 6000 GCOS port in some detail. In 1977, there were three different ports of Unix underway – Interdata 7/32 port at Wollongong University in Australia, Interdata 8/32 port at Bell Labs, and IBM 370 mainframe port at Princeton University – and those three had C compilers too.

[0] https://www.bell-labs.com/usr/dmr/www/portpap.pdf

[1] https://apps.dtic.mil/dtic/tr/fulltext/u2/a010218.pdf (his actual thesis was submitted to MIT in 1974; this PDF is a 1975 republication of his thesis as an MIT Project MAC technical report)

Here’s something cool: the source code to Snyder’s compiler: https://github.com/PDP-10/Snyder-C-compiler

Unix was originally written for an 18-bit machine.

That was before C which is what we are talking about.