Those distfiles… /usr/portage/distfiles can get pretty big, and cleaning it out periodically can save a lot of disk space. The brute force approach is to just clear out the whole thing, which can be more practical if you have a fast connection. If redownloading what you need is no big deal, then cleaning out distfiles can be the faster way to go.
Alternatively, there are some scripts floating around to help you clean out the distfiles (and the other uncleaned up crap) for things that aren’t currently installed (old versions, etc).
- TIP Clean Up Cruft [gentoo-wiki.com]
- Cleaning out stale distfiles
- Portage Utilities Not in Portage
- Experimental Gentoo Scripts
- Filesystem cruft script: clean your system, save disk space! [forums.gentoo.org]