I think it's more to do with mirror power. I aquired a 12" dob last week and am trying to get it onto the mount with a camera down the tube to see if it gives more detail.
It's really hard to get 355mm tube rings
[edit] the Vixen has the reducer in so it's about 1200 F/L @ F6.3 similar to an 8" dob. I imagine the detail difference between an 8" dob and a 12" dob will also show up in the photos.