Originally it was written in lisp then translated using a tool. But I believe it's maintained like that.

When you get into more "low level" domains, huge single files are more common. Some feel there is a correctness justification, being able to follow the code from top to bottom rather than jumping around, see: http://number-none.com/blow/blog/programming/2014/09/26/carm...

Personally I think it just doesn't matter. 50k lines of code is 50k lines of code. Whether it is in one file or 10 or 100, only changes whether you jump around via a file tree or other editor tools.

There are some practical problems when you get too huge though, like not being able to view the thing in github!