Perhaps the sketch would represent something more like a catenary. That illustrates the downward curvature in the middle mentioned before.
When you suspend a chain from two hooks and let it hang naturally under its own weight, the curve it describes is called a catenary . Any hanging chain will naturally find this equilibrium shape, in which the forces of tension (coming from the hooks holding the chain up) and the force of gravity pulling downwards exactly balance.
The links of the chain in the middle go down further, as for the rotors before: indeed they are farthest from any of the two hooks.
And for a same mass suspended (in the middle) from a rope, the longer the rope, the greater the tension required for the rope to be straight.
This is one reason why even in flat terrain two pylons will not be enough if they are too far apart: either the tension on the cable will have to be too high, or the cable in its middle will drag on the ground by cable sag. This is why several pylons are used. These pylons would be a little like lifter kites all the 10 or 15 m.
Some explains about pylons, then a cable sag calculator:
If you multiplied the Straightline distance by 10 (50 m instead of 5 m), and you divided by 10 the cable weight per unit length in order to keep the same total cable weight, the result will be 10x, leading to a lever effect or something similar.
Thus my question remains:
As for my guess:
The forces from the rotors are the drag by the wind, and the gravity by their weight (mass), but not the lift by their own (without cyclic control) except perhaps the lift aggregated to the main lift from the lifter kite.
So I guess that as they scale, shaft sags occur, leading to the requirement of a lifter kite which ends to be too large, or (perhaps yes, perhaps no) to the requirement of relatively wider shafts (and rotors) compared to the length.