"One Size Fits Most" is a popular size for many lingerie items. It's not one size fits all.
These items tend to have a lot a give, so it's important to use the size chart provided on the item description page.
The sizing for most one size products is the following:
1. One Size Fits Most items are suggested to fit 90lbs - 140lbs. If you are XS, SM, MED, and sometimes LG, one size fits most should work for you.
2. One Size Fits Most Plus or Queen items are for the curvaceous ladies. It's suggested to fit 160lb - 225lbs or a size LG, XL, 1X, 2X, and sometimes a 3X.