The cups are just about the same size, maybe a few millimeters difference. The drivers also appear to be the same size.
I wish I had a pair of HE90's to compare to the Jades. I'm not saying they sound the same but they are a lot closer than the O2 is (not that the O2 was meant to sound like the HE90). As good as the O2 is with the right amp, to my ears there Jade are just plain better. First, as PJ mentioned, in terms of comfort, the Jade wins, end of story. The soundstage/imaging/airiness of the Jade is better than the O2. Speakers are my first choice most times and as such soundstage, etc. is important. They extend well in both directions. The bass is deep and controlled. It produces detail that is almost palpable. The Jade impresses me now as the first version did in MOA. The BH just makes them sing. This pair has about 8 or 9 hours on them at the moment so hopefully this will continue and improve over the next week or so.