Since IIRC bug count is roughly proportional to source code line count, this implies the aggregate complexity of software doubles too. That might sound foreign, but if you think duplicated effort and accidental complexity rather than problem domain complexity it wouldn't surprise me if it was close to true.
Points to the value of sticking close to the problem domain.
I heard error count is proportional to LoC + # of developers involved in writing it. So as well as the accidental complexity from LoC, the more people touch a code base, the greater the chance of broken communication and misunderstandings. Ie bugs.
Maybe. In my experience it's common for developers who were not part of the original effort to maintain by addition, because they are worried changing the existing code might break stuff. So with increased maintainer count the code size grows too.
Points to the value of sticking close to the problem domain.