Thinking of the question from a different angle - the wing needs a range
of speeds to give good performance (lift versus drag) and the weight
dictates how much lift, and therefore speed, will be needed. An
extremely light load will need less speed to create enough lift, but
below a certain speed, the wing's drag vs lift will get in the way, and
though it is going very slowly, both forward and down, the glide ratio
may not be all that good. As weight increases, the speed necessary
increases as well, and there is a best combination of speed vs lift vs
drag for best glide ratio. Excessive weight will demand yet more speed,
but drag increases as well, so the glide ratio suffers. Our theoretical
balsa versus lead U2 might, ironically, have not dissimilar glide
ratios, while the real thing at an intermediate weight, have the better ratio.