I don't quite follow this logic. If one manipulates a codebase in the ways you describe (merging with another, or dividing a larger one), the result doesn't eve

I don't quite follow this logic. If one manipulates a codebase in the ways you describe (merging with another, or dividing a larger one), the result doesn't even have to be a working program; how can we compare the result with the original? The real answer is probably that it might depends on how tightly coupled the code is, how much test coverage there is (assuming that tests are included in code size), the quality of the authors who wrote the code (this might be circular), maybe even on the technology used, or what you consider distinct bugs, etc. My intuition leans towards super-linear, but only slightly so -- large codebases tend to have many authors (many of whom may have since stopped working on the project) and be old.