Linux is a big
heap of bloat. It seems every UNIX is, too.
My definition of light-weight is floppy-based, sub-4 MB.
For PCs that leaves a few things like BeOS, Minix, DOS,
ConcurrentDOS, CCPM and not much else.
I think you're thinking QNX and not BeOS (BeOS requires at least 32MB and
200MB disk space). I know QNX can be squeezed down to that (or at least
could at one point, not sure if it still will fit on a floppy).
Zane