The weight we give for a basket size is the nominal dough weight. That is, all the ingredients mixed together including the flour and water. it is a guide though - some doughs are denser or livelier than others and will occupy more or less volume when proven. The values are pretty good though and have been in placefor many years.
If you want to find a basket size for a specific amount of flour, a rule of thumb is to multiply your flour amount by 1.7, so a loaf made with 500g flour will have about 350g water (500x1.7=850g). 850g dough should be proved in a 1kg proving basket or banneton.