The wings are not sagging, but have what is known as Anhedral. Western civil aircraft have dihedral, which is when the wings seem to point upwards along their span. This is an inherently stable design and I will illustrate this in a moment.
The previous posts have already mentioned the military aspect of Russian aircraft design, in particular civil airliners. Illushyn 62's, Tupolev 104, 124, 134 and 154 are all civil airliners with anhedral. Western military aircraft also have anhedral, so why?
A military aircraft must have rapid response to the controls for maximum manouverabiity in a conflict situation, therefore an unstable design will assist in this respect.
The best way to illustrate the differences between anhedral and dihedral is to imagine a small ball cut in half. One half has the open side uppermost and the other the open side down.
The half with the open side up we will call dihedral. Now imagine placing a pea in this half. The result is that the pea finds it's way to the bottom and remains stable.
Now with the open side down half which we will call anhedral, placing the pea on the top of the ball will require skill to prevent it falling to one side or the other. It reasons therefore that the slightest movement will cause the pea to roll rapidly, where as the stable pea in the dihedral half will require a much more determined movement to make it move. Also this pea is easily returned to a stable condition, where the anhedral pea is not.
The Boeing 747 with low wing dihedral and underslung engines was designed for civil use from paved runways, but the Lockheed C5A Galaxy with a high mounted anhedral wing was designed for military use.
Going back to the Russian designs, I can only assume that their civil airliners were designed with massive troop transport in mind during the Cold War years.
I hope that explains it well enough, just think of the pea as an aircraft fuselage.