The amount of ram a software uses seems like an odd metric to use, especially when the ram hungry beasts mentioned in the article use less than 250 MB.
IMO, the fact that they require more memory can be considered a reasonable indicator that they are more complex / do more stuff. They need to read stuff from disk / generate data in order to fill that memory in the first place, which takes time. I'm not an expert, but I would guess that using more memory also increases the chance that some of it will be swapped to disk by the OS, slowing things down further (although only intermittently).
Comparing IceWM and JWM seems fair, because they both do the same thing. Comparing JWM with Gnome3 only on memory used for a vanilla instal is weird.
Besides, it's interesting to see that E17 is hardly 'lightweight', and XFCE is actually 'heavier' than MATE. WindowMaker (woo hoo!) is surprisingly 'lighter' than Awesome. I'm glad it will cut down on the X-is-slow-use-Y-instead-it's-lighter crap. Hearsay and fanboyism are endemic to linux wms, it seems.
You and I know as geeks that irrespective of real-world use, metrics and numbers are a good thing to throw around and make us seem smarter and more efficient than we actually are. :D
I was intrigued by the results - but will still keep using Unity myself (I love Mate/Cinnamon too) though its second last!