I've always had that issue with Safari. I've tried other browsers and settled with Camino. Little memory usage, depending on sites I visit, but less RAM usage than Firefox and Opera. Opera is my second choice. I just don't like dealing with the increasing memory usage, beach ball spinning and the slowness of Safari. I have to use it for certain sites right now as I need a fresh installation of Camino because something has gotten corrupted. Verified by a free app called Troubleshoot Camino. I also have a paid app called iFreeMem that frees up RAM without rebooting. Works great for me.
I've also been using a free (donations accepted) app called Memory Monitor for years. It uses less memory than Activity monitor. Bernard Baher made it.
http://www.bernhard-baehr.de/
I used to use Mac Janitor to clean up things & release memory. It won't work properly on Leopard, but it may still work with Tiger. It's free also. It worked better than iFreeMem. Both take a miniscule amount of time to work.
Hope this helps you. These apps did help me tremendously.