Your experience doesn't match mine. I have, mostly solo, and part time, written multiple codebases that on that kind of magnitude (it is about the level where it still will fit in one person's head pretty easily IMO). It doesn't take much to reach that kind of size. Now, if all of it was super dense and subtle code, then yeah, that would be a lot, but in my experience that's usually a pretty small part of any given codebase.
> in my experience that's usually a pretty small part of any given codebase
Our experiences differ then. Mine is that almost all of the code I write is directly targeted on the usually quite complex problem I am trying to solve. I don't do boilerplate, for example.
I tend not to have much boilerplate (and write abstractions to avoid it), but I do still find there tends to be a lot of supporting code around the 'difficult bits' (TBH, most of the code I write is supporting a small amount of relatively simple but subtle operations, but such is the nature of embedded software). But different codebases are quite different in this regard: this is why such different scales shouldn't be too surprising in different domains.