When I wore a corset almost around the clock for about three years after an accident, my restricted waist shrank to 65 cm (about 25 1/2") and my unrestricted waist to around 68–70 cm (27 1/2"). I think it also shaped my ribs somewhat. (My current waist, years later, is about 74 cm, or 29".)
As Mavis also said, I found the point to be not to relax into the corset, but to rather use my own muscles to maintain a good posture independently of it. That really strengthened and toned my midriff and also eliminated the occasional back pain I sometimes had even before the accident.