See this post: Delete files automatically - CAB files and CBS log issues
This is really a Microsoft issue, and less so a BigFix issue, though BigFix can be used to help solve the problem.
As far as I know @JasonWalker has the root cause, but you can research this issue online generally on Microsoft forums and related sites.
You could probably take the current CBS.log file, use 7zip to compress / archive it, then delete it so it can start fresh.
Then delete the cluttering files that are eating up space.