Comment 2 for bug 21821

I haven't changed that preference from the default (delete packages that are no
longer available).

It's not space consumed by the cached packages themselves, but temporary space
consumed during operations that is not being freed. For example, I just closed
synaptic after it had been running for a day or so, and I immediately got ~200MB
of free space back, while the size of /var/cache/apt didn't change. I only
noticed this bug because I had left synaptic running long enough (with repeated
reloads and upgrades) that / (I just use / and /home) ran out of space. As soon
as I quit synaptic I got almost 1GB back.