It seems like Venix/86 came out in Spring 1983 and PC/IX in Spring 1984. I guess by then RAM had become cheap enough that running in 64KB of core was no longer a requirement and LSX and MX did not make sense anymore. Does that sound right?
Venix/86 2.0 (still 7th edition) requires 192k to run, at least on my Rainbow, and get to login:. 128k and 64k simply are too small configurations to run it. There's not a lot of 'fat' in the Venix kernel and more-modern compilers only are able to make modest gains over the primitive pcc used at the time. The raw kernel for pc/ix is a few k larger than the venix kernel. So I'm guessing that's right.
Warner