I just noticed this but when I start up a fresh instance of pl/sql developer it immediately uses 71mb of memory. After I was using it for a while it was up to 130m. Version 9 is usually around 10m - 15m.
Did something change to now use more memory? Is this expected behavior now?
For me 10.0.2.1697 on XP SP3 is using 30 MiB when showing the logon dialogue and 60 MiB when showing the main window. The only tool windows that are open are the object browser (no entry unfolded) and the window list (no entry).
I see 43MB after starting PL/SQL Developer and logging on, both for 9.0 and 10.0. It may depend on the Plug-Ins you have installed, your Oracle Client, and other environment factors.