In works of art, mass exists in one of two forms. Either the work has actual weight or it has implied weight. If it it has actual weight, then the mass is the weight of that object (sculpture or architectural structure). If it has implied weight, then the actual weight is not as releveant. When viewing a painting few consider how much the paint, canvas, and frame weigh and are more focused on what the objects within that painting might weigh if they were the real objects the work is suggesting.
One other consideration related to mass is that of volume, and in the arts is mostly restricted to interior space.