On my x-wrt install, I have a ~140Kb list of WhiteRussian packages, a ~247Kb list of 0.9-backports packages and a ~200Kb list of X-Wrt packages.

That's pushing 600Kb on a 4Mb router   - Even compressed, that was enough to stop asterisk installing.

So I deleted the lists, made a temporary folder, and a symbolic link from the old list location to the temporary folder and a bit of tweaking:

*** Warning: This will lose the package lists after every reboot and require directory recreating to fix it ***

cd /jffs/usr/lib/ipkg
rm -rf lists
mkdir /tmp/lists
ln -s /tmp/lists lists
reboot

The reboot is for jffs to refresh and show the freed up space. There may be a neater way than that, but that works. I then had ~1.6Mb free instead of ~1.2Mb.

After rebooting, the symbolic link is broken because the tmp filesystem is lost during a reboot. Still, we can recreate it:

cd /jffs/usr/lib/ipkg
rm lists
mkdir /tmp/lists
ln -s /tmp/lists lists

Now you can refresh the lists:

ipkg update

And then retry installing whatever it is that you wanted.