Originally Posted by
PepiMK
Well, inside the installer, the files are all compressed, so it can be expected they're a bit larger once installed. And of course there is the detection database, which gets downloaded afterwards (the installer would nearly always be outdated if they were included). That plus the archives of the update files (which are kept to allow users to easier reproduce what the application is doing during an update, even if it looks like a waste of space at first) could easily make the 20 MB I guess.
Then logs (1 or 2 MB maybe), the initial registry backup (can easily be 10 MB), probably the quarantine (totally depends on how much stuff you've removed with Spybot-S&D)... I don't think the Add/Remove page will count them, but they reside inside the Application Data folder.