IE takes up about 15MB PER open window on my computer. Mozilla Firebird takes up 34MB, but that's including three windows open, one of which currently has five tabs open in it, all on separate windows. Opening up all of the same things in internet explorer takes about 105MB of memory.
Wow! I hadn't noticed that. My not-very-controlled experiment: open up an IE window for the each of the exact 4 pages currently open in Firefox in different tabs, and open 4 identical tabs in Opera 7.23; report memory usages:
Firefox Total: 42020k Opera Total: 37268k IE Total: 65460k
Not as dramatic a case as yours, but IE still comes out looking like kind of a hog...