Earlier, Steve asked why, with the extra features (full zip, baffles, etc.), the WM Summerlite weighed less than the Atom. The shell material and down (quality and/or quantity) could be part of it, but the cut of the bag could also be a culprit.
Reading the specs, it appears that the WM is a slimmer fit than the Atom. I don't own either the Atom or Summerlite bag, but I know from crawling into the Marmot Hydrogen and Helium in the shop that they are roomier than most of the comparable WM bags. I prefer the WM bags to the Marmot, if they fit; if they didn't, Marmot would be my first alternate. (For what it's worth, I ended up with the WM Mitylite and Megalite; the other bags I tried in the Extremelite series were cut too tight for me.)