Hi Greg,
Quetiapine can be used for a number of different symptoms. As I understand it, medication in this group can help with many of the symptoms associated with anxiety and these would include obsessive thoughts and derealization and depersonalization.
I really wouldn't know what dose would actually put you at risk of weight gain, but I would hazard a guess that it would be much higher doses than we're on, if you look on your Quetiapine leaflet, you'll see just how high the doses can go. For the likes of me and you or, indeed, anyone using it for anxiety symptoms, it's only ever given in very small doses, so I would think that we're at the bottom of the risk list for side effects.
I would carry on your healthy life style, wished I could be so active!! and just keep an eye on your weight if it worry's you, or have a chat with your doctor, I'm sure they'll be able to put your mind at rest