I agree. XUL isn't that bad, but it seems like XUL based applications use about twice as much RAM as their non-XUL counterparts. Firefox and Thunderbird are fairly competitive on that front with other web browsers, but Songbird easily uses over 100MB of RAM, and is sluggish, whereas even Windows Media Player, which for some reason has a reputation of bloatedness, uses under 30MB on my machine unless playing video, which Songbird can't even do.

That said, I think one of the advantages to XUL is that it allows for easy modifications and extensions. On the Firefox/Thunderbird front, that's worth it, but in some other cases, it might not be. I think even Amarok on Windows (when it works) uses less RAM than Songbird on Windows.