To add to Alex's post.
The density of E-glass is 2.54 g/cc, and the density of laminating resins range varies from 1.15 g/cc to 1.35 g.cc depending on the specific epoxy formulation. For a standard woven fiberglass cloth using a 1.25 g/cc epoxy, the ideal epoxy to glass w/w ration is ~1:1. (Actually 98:100 with a range of 90:100 to 106:100.)
The density of Carbon Fibers vary with grade. Commercial grade T-300 fibers have a density of 1.76 g/cc. Higher strength spec fibers have a higher density which I have not looked up.
T300 Fibers (568Ksi, 34Msi)
AS4 Fibers (640Ksi, 34Msi)
T700 Fibers (711Ksi, 34Msi)
IM7 Fibers (820Ksi, 40Msi)
Since the higher strength grade fibers have higher density, the lower weight % of epoxy is require to make a optimum composition composite however the tightness of the weave will influence the % epoxy in any composite.