Chornedsnorkack, assuming all engines all equall tchnolgically advanced, a big engine (compressor / combuster / turbine) is more efficient then a equally advanced smaller engine. That why the A350 / 777 / 787 have two in stead of more engines.
Apart from that I think the aircraft types you mention developped during the last 50 yrs where not dealing with oil prices approaching $100 / barrel.
That makes it likely new developments next to the old ways of designing aircraft become more feasible.
I did a comparison of the same aircraft equipped with 4 new engines, it doesn't seem like a good idea :
Note that a tripple engined MD11 version would have 3 engine + 1 APTU = 4, while this aircraft has 3 powerplants with associated wiring, tubing, structure etc.
About the thin side wall, lets remind ourselves these are only conceptual sketches, measuring the pictures and commenting is't usefull IMO.
