Their claims was that lxde uses 1/3 the memory of XFCE4. I didn't believe it, so I conducted my own set of tests to debunk this myth.
Here is a test I conducted just now. Using my Laptop, Acer Aspire 5315, 1.73 ghz celeron, 2 gigs of RAM. Ubuntu 9.04, cli only install, xfce4 installed from apt. Very lean installation of Ubuntu, runs great on a 10 year old Compaq notebook as well.
Anyhoo, back to the test. I created a test user. I turned off the display manager, and rebooted the laptop before starting each of the desktop environments. Here is my results:
free memory after reboot: 1,944,304
free memory after startxfce4: 1,789,552
free memory after startlxde: 1,804,456 ( Yes, I rebooted before I did startlxde )
xfce4 difference: 154,752
lxde difference: 139,848
Difference between xfce4 and lxde: lxde uses 15 less megs of ram than XFCE4.
I'm sure I would get different results if I used a different machine, but I think you catch my drift. LXDE isn't that radically smaller than XFCE4.
UPDATE: Did the same test just now on my Compaq Presario 700. Athlon 500 with 256 megs of ram. Same Ubuntu install, same packages.
free after reboot: 163,868
free after startxfce4: 92,312
free after startlxde: 107,320
Again, difference of 15 megs in favor of lxde. Doesn't seem like enough to worry about to me.