What a load of negativity and nonsense.
Even conceding for the sake of argument your point is valid, this particular dress isn't even a tight fitting, body con dress. It's intended to hang loose on the model and if the wearer wears the right size it'll hang loose on a size 8 or an 18 or a size 10 or a 20.
A size 20 trying to squeeze into a size 16 isn't going to look great. A size 20 wearing a dress designed and made in a size 20 - no issues.