I think that most people in size acceptance would rather shoot for acceptance than a smaller size; if people of all shapes and sizes are treated equally, then the quest for thinness becomes moot. But I don't purport to speak for everyone, so speak your mind.This has always been true, although sometimes I've lost sight of it: I don't have any problem with my size. My problem is other people have a problem with my size. Parts of the world I inhabit aren't set up for someone of my size. I can't fit into some furniture, and I am constantly reading fat-hysterical, fat-hating stuff.
If I could make one wish related to my size, it would be that the world treated people of my size as normal. Furniture, toilet stalls, medical equipment, workout equipment is made to fit us, we show up in the media and are considered attractive and healthy, the medical issues we do tend to have are considered ordinary medical issues that normal humans have, clothing comes in all the varieties of shapes we are and even business clothing stretches to fit us when we're sitting down, showers come standard with scrub brushes for our backs, etc.
The fact that all of those things aren't true in my culture constitutes a constant reminder of fat people's second-class status. That makes it more of a struggle to maintain one's self-esteem and remind oneself that I'm not broken, the culture is simply lacking in accommodation.
I've made plenty of accommodations to the world - I'm on antidepressants partly because the world doesn't appreciate hermits, I dress in ordinary clothes and wear an ordinary haircut partly because the world doesn't have enough imagination to appreciate what I'd prefer to wear, and so on. I'm not interested in changing my body shape to suit the world.
On the other hand, if I could take a pill to change my body shape and size temporarily (as in for a period of a few days or weeks), I'd probably do so in order to experience some of the things I physically can't do with my current body, such as mountain climbing, or certain sexual positions. But I'd also take a pill to change into a cat so I could leap five times my height with no effort and have swivel ears. So that's not about wishing I were thinner per se.