Since there are no surviving garments and no surviving descriptions of what was worn, the only evidence for their clothing comes from small statues and some wall art. To make it more difficult to understand these images are not of "everyday" women, but often of goddesses. So it's even harder to say if the representations we can find are even of real garments or "real" people. Unless some miracle extant reference is unearthed there's very little that can be assumed beyond guessing. Later corsets/stays/bodies are sometimes stiffened with cane, wood, grass, or even just gum-stiffened fabric, which would all be available much earlier. Leather would also be possible. However, from the way the bodice cuts below the bust, and that the few examples that show a back show it cut very low, it's not likely this bodice would have had much effect on the body. It may have been stiff, it may have been "tight", but it's not likely that it made much impact on the actual form. It wasn't until the late 19th century that corsets were (occasionally) used for that purpose, though documented bodies/stays were common as of the late 16th century. It took quite a bit of development in shaping and patterning to achieve that. I'd also argue that the figures that are particularly narrow waisted are not exactly the most anatomically correct, based on the shape of the breasts alone, so it's hard to assume anything based on that.