A reasonably comon proportion of glass thickness to wood is 20/60/20. Any time you have something like a tube where the walls are 20% of the thickness of the tube, that portion of the tube is doing almost all of the stiffness work, a little under 80%.

Wood actually has some significant advantages over fiberglass where stiffness to weight is concerned, and particularly where fatigue is the issue, but it doesn't like the levels of deflection that are present in bows, which is why it take a 1 in 100000 piece of wood to make it into a bow, and it still doesn't last for ever.