File sizes: Avast scans more data than the file has???

Ok. So I downloaded a Wesnoth installer (http://en.wikipedia.org/wiki/The_Battle_for_Wesnoth) and scanned it by Avast Free. Windows tells me that the installer has only 298 MB, and when I tried unpacking it with 7zip it had over 400 MB. But Avast told me that it scanned over 700 MB! That’s a nonsense! Also, if I’m not mistaken, after I scanned the whole HDD Avast claimed that it had scanned more data then my HDD can store - I usually have ~30-40GB stored on my Windows partition (there’s also a Linux one, because I tried experimenting with it a few months ago), and it can store up to 45.4GB, but, if I rember well, Avast claimed that it had scanned around 60 or even 70 GB! I’m totally confused with it.

Sorry for my English and thank you in advance.

It reports the size of all the unpacked data. Even stuff that’s packed with compressors no archiver can ever unpack. These days almost all executables are packed to decrease size.