Generally wings that are large and have wing mounted engines tend to be soft ride wings. The wing mounted engines strengthen the wing therefore
it tends to have some more wiggle room. Think 747-400
wings that are clean(no mounted engines) tend to need reinforcement
and are stiffened. Think MD-80.
There are other design characteristics that help in the design of soft and stiff wings but they escape me, too technical for my noggin.
Happy boxing day