I think there are a couple of different phenomena being discussed here though. I think the original post referred to the belly of the MD
-11. Later mention of the MD
-11 / DC-10 upper rear fuselage below the no. 2 engine refers to a design feature, as n901wa says, which is one of the type's signature visual characteristics.
Personally, I don't see a "bent" fuselage belly profile on the 757, and suggest that what can be seen is the parallel but unaligned belly profile ahead and aft of the wing. The rear fuselage is a deeper cross-section that the forward i.e. behind the wing/fuselage fairings it doesn't return to the same depth as the fwd fuselage. The 727 is the same.
However, the 777-300 pics are interesting. Displaying them large and placing a ruler on the screen, there does seem to be a mis-alignment of the belly fwd vs. aft of the wing. It does appear bent. The area for scrutiny is near the centre of the frame in each of the 777-300 pics, taken with telephoto lenses, so there's minimal peripheral distortion, so brilliantly illustrated in aviopic's wide-angle Caravan image.
Other candidates for this phenomenon may include the A340-600 and DC-8-61/3/71/73 so I'd look for long focal length, side profile shots.
I once heard that large ships e.g. supertankers are built with a bent hull to take account of the curvature of the earth, and know that the towers of suspension bridges (e.g. Humber, Severn, Verrazano Narrows etc.) are out of parallel for the same reason. However I would not think this is a consideration in aircraft design!
Regards - musang