Well, bloat is somewhat subjective, so "evidence" is hard to come by. I don't know about the other poster's reasons, but I agree, so here are mine. That xulrunner/FF/thunderbird/etc. are built from the same tree is the first indicator. Then there are the multitude of build options, a few pages' worth. The output of these two commands is somewhat telling:
~/src/mozilla-1.9.2$ find -name \.c -o -name \.cpp -o -name \.h -o -name \.idl -o -name \.js -o -name \.xml | grep -v /tests/ | wc -l
11740
~/src/mozilla-1.9.2$ find -name \.c -o -name \.cpp -o -name \.h -o -name \.idl -o -name \.js -o -name \.xml | grep -v /tests/ | xargs cat | wc -l
4386362
(Painful to construct command lines like that on a phone, and that is a somewhat old version of the tree, but unless FF4 is significantly smaller than 4m lines and doesn't require building xulrunner and FF, the point stands.) The large number of dependencies is another indicator.
I'd be remiss if I didn't mention that FF is still my "workhorse" browser, and I like it a lot, despite the flaws and the bloat. (I've been using Arora for a long time, but it has some stability issues still. I like Luakit, which is small and hackable, but it's kind of a niche browser. I'm not at all a fan of Chrome.)