There's the difference. Given a thousand line script, I'd easily clean it up and make it work properly betting on the time saved in maintaining it. For ten-thousand line scripts, I'd question the source for having one script that long and be more hesitant to maintain it. Is that really a small script with many modules? That might be okay to maintain or it might be trouble.
A million line system and one person maintaining it? That sounds like a job (or a nightmare) all by itself. Are there many independent modules that don't need to be changed? At best, that would be difficult.