OMG it was an extension, from Active Helper, I tested their online help system. Seemed fine to me till now. It made me think that the file is bloated in all my backups, 8 days, but the problem was not present 3-4 weeks ago. And the only thing I did in this time frame was installing this component. I should try to report this somehow... thank you for your interest.
After removing the comeponent and the module the behaviour doesn't occur anymore.