Size generally refers to volume in respect to containers. As a
result a bag filled with nails will have more mass than an equal
sized bag filled with cotton. Inertia is proportional to mass and
the velocity the mass is travelling at, or alternately the mass and
the acceleration it is subjected to. As a consequence a bag filled
with nails travelling at the same velocity (or subjected to the
same acceleration) as an equal sized bag filled with cotton has a
greater inertia due to its greater mass.