I could buy a similar argument for directories: you will almost never see a C project with sources in subdirectories of the top-level source directory -- this i

I could buy a similar argument for directories: you will almost never see a C project with sources in subdirectories of the top-level source directory -- this is because of the recursive Makefiles which earned quite a bit of a somewhat justifiable hate. But I don't think compilation times explain the size of the source files. This hasn't been a problem for such a long time that I cannot even remember when it could have possibly been a problem. I had seen the reverse problem, but not with C... rather with Python source files. The older parser used to be very bad and would start using too much memory if the source file was in the thousands of LOC. I had to witness this firsthand with SWIG-generated Python bindings. I don't remember this kind of problem with C compilers / other utilities though.