I would guess that for most people, it's not visual in that way at all. It's more a proprioception-based concept of your shape and your presence in the world, and where you are in relation to the things around you.
I was quite thin for most of my life, and that shaped my internal construct of where I am in relation to things around me. If I don't think about it, my "instinct" is that I can totally fit through that narrow gap. I can't, because I'm quite a bit more round than I was 10 years ago. But my mental construct of myself hasn't caught up. Same kind of thing with how flexible I think I am, or whether or not hiking up that steep trail is going to leave me winded. My instinctual concept of myself doesn't conform to objective reality