See attached screenshot. The size seems a bit absurd. What's this file being used for anyways?
Is something writing to that file (eg your logs are getting piped to it?) I normally delete that file and the UPGRADING file as you don't need them once you have setup SIlverStripe.
I have no idea, I haven't set up anything to log to that file. I'll remove it now that you've said it isn't necessary to keep.