Flexible bags having stretch-to-fit conformity to closely accommodate contents in use

A flexible bag comprising at least one sheet of flexible sheet material assembled to form a semi-enclosed container having an opening defined by a periphery, said opening defining an opening plane, said bag being expandable in response to forces exerted by contents within said bag to provide an increase in volume of said bag such that said bag accommodates the contents placed therein, said sheet material including a first region and a second region being comprised of the same material composition, said first region undergoing a substantially molecular-level deformation and said second region initially undergoing a substantially geometric deformation when said sheet material subjected to an applied elongation along at least one axis.