This is an odd phenomenon. To know exactly, you'd have to read the error file. And I know that's not easy, you'd have to FTP the file probably, and then open it in a text editor that can handle that size file.
Short of that, I have an idea for you. A site I've worked on had a similar problem, and eventually the file would get SO big that the site would stop coming up. So I read the huge error file, which pointed me in the right direction. WP was running out of memory. Here's a nice article about that if you'd like to try this approach:
For me, just using the first command fixed it. Beyond that, you may need some help from host tech support unless you're a server geek.
But ultimately, yes, you'll want to delete that monster error file, hopefully after finding out what the error is.
Good luck, Dave