Perhaps Craigster can chime in, not sure if the the SigDJs have the same range of motion, but Ultrasone's PRO 900 headphones fold up seemingly as small as the M-100, while also folding flat.
Does anyone have experience with the Pro 900? It's in the same price range, and even looks a bit more comfy. I don't know a single thing about it other than quickly checking it out on the Ultrasone website.
The Signature Pros and DJs will fold up like the M-100s too but that picture the cups are :-
1) turned inside out
2) length extended to the max
3) and purposefully compressed