Hi and welcome, urit,
Do you have the option to scan "archives" (compressed files) turned on? My first guess is that you do -- try turning that option off.
When avast scans archives, it needs to uncompress them to "see them" better, and of course will require additional disk space temporarily for this. They may be some totally different reason for your problem, but if you're fairly tight on disk space to start with then I'd bet the archive-scanning is at least partly where your problem is.
More experienced tech help will be along shortly, I'm sure -- but that'll at least give you one place to look.
Best,
Mike