> libraries are used throughout the codebase as vocabulary types. Again, I think this just comes down to very different codebases and uses of libraries for us.

I don't know what 'vocabulary types' are, but I think this is an oversimplification. My guess is that most big programs end up like yours but my point is that it doesn't have to be this way. Single file libraries are not the problem and can actually help, although the real issue is deeper.

I think the problem is actually dependencies and many times single file libraries go to great lengths to not depend on anything else. To keep it as simple as possible, I think classes / data structures get over used and instead of just making an interface to some data, people stuff everything they are doing into some class they have.

Transformations from one type to another put into data structures means that the class now has dependencies on those other types. If those types have transformations then they have dependencies and so on. The extrapolation is that everything depends on everything else and even compilation units that should be tiny end up pulling in large parts of the entire program as source code. Lots of compilation units can then mean compiling huge chunks of the source hundreds of times.