Rootfs is compressed and thus not immediately flushed. Reboot before taking measurements.

Increases in messages, calls, etc, got logged. Files change dynamically.
Repositories grow. (Yes I read). Just saying.

Several apps that seem very small depend on libraries that hare huge. For example, Google Latitude is 6M, but it pulls libqt4-network that's 21M and not optified apparently.

Rootfs varies, sometimes as much as 10 MB depending on repos, boot, upgrading packages, etc.

I, e.g., have some 30 MB left, with ABI installed.

I warmly recommend you install Storage Usage and scan packages. It shows you per-app usage, red for non-optified, green for optified.