I gave a quick answer but this question deserves deeper thought because I think it is a product of various ideas about fatness, including:
- Fat people would rather be thin because obviously it's better to be thin
- There is nothing of value in being fat
- Bodies are choices therefore transformation is desirable and possible
- The fantasy of transformation from an unbearable present to a beautiful future is preferable to the struggle to make the present liveable now
- Disbelief that embodied self-acceptance is possible
It won't come as a surprise, but I don't subscribe to these ideas in relation to my own body. However, I do fantasise a lot about things. Often these are impossible supernatural things, like waking up and all the dead people I love are alive again; or about a more prosaic sadness, like waking up to find that I am adored and worshipped by someone who plainly doesn't adore or worship me in real life. Sometimes I dream about things that appear impossible but become more feasible when I pick them apart; waking up with unimaginable wealth is actually the desire to be able to do what I want with my life, to be able to help other people whenever I like, and not having to worry about paying for something, rather than sitting on a yacht with a bunch of supermodels. I think dreaming and imagining are crucial for anyone with an interest in social change; fantasy and desire, an imagining of something different, they are the first unformed, free-flowing steps to taking action.
Occasionally I harbour fantasies of having a different kind of body. Having a tail, a penis, the ability to project my thoughts out of my eyeballs like a film, or type anything just by tapping my fingers on a surface, being able to fly, to breathe underwater, to teleport, to grow or miniaturise myself, this is what I think about. I also wonder what it might feel like to have different impairments like some of the people I know. The fantasy of waking up thin is not really there, it's too boring, I just don't value slenderness in that way.
When I return to the original question I think I am being asked about the circumstances under which I would want to be thin. It boils down to this: it would be interesting to take advantage of the capital and privilege that comes with being normatively embodied for a couple of days, though I think this would enrage me to a level where I would struggle to function. I think Linda Bacon's 2009 NAAFA keynote, Reflections on Fat Acceptance: Lessons Learned from Privilege (.pdf, 128kb) is really central to the reasons why fat people might fantasise about becoming thin, it's about acessing power and privilege. This explains why people would risk the drastic and uncertain steps to realise normatively thin embodiment, and why this is problematic for anyone who cares about embodied diversity and the unequal distribution of power.
The question about waking up thin comes from a series of common values and beliefs that have become very alien to me and it's fun to take them apart and answer the question from my current standpoint. I'm intrigued, for example, by the way that very thin people can scooch up into tiny spaces, so I'd like to try that for, I dunno, 20 minutes or so before I returned home to my own body. It's a funny reversal of the fantasy, being thin is the temporary state in my mind's eye because when I think about it, who would I be without my body and my own history of fat embodiment? There's no way I would ever want to deny that precious stuff.