A fan blade has an airfoil main body extending between a leading edge and a trailing edge. The airfoil also has suction and pressure sides. A cavity is formed into said main body, and receives a filler material. A cover closes off the cavity, and is attached to the main body, with the cover having a thickness defined in a direction perpendicular to the suction side. The main body has a spar which extends along the cavity, with a thickness of the spar at a central location between ends of the cavity which has a second thickness. A ratio of the first thickness to the second thickness is between.5 and 2. A fan rotor and a gas turbine engine are also disclosed.