Actually, a wing with more weight doesn't need to be as strong as a wing with less weight. Let me explain. When in flight, gravity is acting on the fusalage, pulling it down (obviously). The wings create lift, and as a result, the wings appear to 'bend' upward, as you may have seen during a heavy takeoff. The fusalage is actually sagging between the wings. The 777 and 747 are famous for this effect, although just not as visible on other aircraft. Now then, when in flight, the wings want to bend upward (as explained above), so the engines, fuel, and weight of the wing itself 'push' it down. Therefore the more weight (in the form of engines) helps keep the wing down, whereas a wing with two engines doesn't have as much weight, so it needs to be stronger to keep the wings from ripping right off. If you have the chance, look at a 747 center section when being assembled. It is the strongest part of the aircraft, and can handle the weight of the engines and fuel with no problem. Also, the wings have braces that run parallel to the fusalage, to facilitate the 'bending' by making the wing more flexable. Seeing that this is not my area of speciality, I did the best I could to explain it. I hope this helps. Perhaps someone who is more articulate and educated on this topic could fill in the gaps. As for the 757 question, I have no clue. Sorry.