depends on the designer, and the piece, Some are just sized, others to keep the preportions correct are scaled up and down, When I do a design that is going to have a big run I do 3 masters.
One for sizes 3 to 6, one for sizes 7 to 10, and one for sizes 11 to 14, that way any of the designs can be sized up or down a size or up too two with out looking like they have been sized and too keep them from looking funny.