As projects surpass one million lines of code, there’s a direct correlation between size and quality for proprietary projects, and an inverse correlation for open source projects.
The article gives numbers: above 1M LOC, defect density increases for open source projects, and decreases for proprietary projects.
Increasing defect density with size is plausible: beyond a certain size, the code base becomes intractable.
Decreasing defect density with size is harder to understand: why should the quality fairy only visit specially big proprietary projects?
Perhaps the way those proprietary projects get into the MLOC range in the first place is with huge tracts of boilerplate, duplicated code, or machine-generated code.
That would inflate up the denominator in the defects/KLOC ratio.
But then that calls the whole defects/KLOC metric into question.