A big part of the reason for hanging the engines is actually because it means you can make the wings less robust. If you have in-wing engines they generally ha

A big part of the reason for hanging the engines is actually because it means you can make the wings less robust. If you have in-wing engines they generally have to be near the root (where the wing connects to the fuselage) because engines are large, wings taper and wings shouldn't be too fat. Under wing engines can be hung wherever you like, within reason. If you simplify things a bit you can think of a wing as a cantilever which supports half the weight of the plane, balanced by an equal upwards force spread out along the whole wing. By shifting weight towards the middle of the wing, as you can do by moving an engine outboard, some of your weight is no longer at the end of the lever but instead is closer (on average) to where the upwards force is being applied. This decreases the load you have to design for even though the weight remains the same.