I use the attached perl scripts (modified from others posted here) to do this - they'll report all packages using rootfs space, and how much they're using (or how much space a given package is using, in total or per file).

EDIT: I've updated the attachment. The new version now reads the dpkg data files directly instead of calling dpkg (which gives an enormous speedup), and also reports the display name as well as the package name (where available).

EDIT2: I've updated the code again, removing the hardcoded non-rootfs paths and instead using stat to check the device IDs for the rootfs and the filesystem the file is on. This means it will deal with any bind mounts, symlinks, etc. transparently. I've also moved the scripts to my webspace rather than having to keep them updated here. They can now be downloaded from http://www.robinhill.me.uk/maemo/package_sizes.html