The glide ratio of an aircraft is often strongly correlated to the efficiency of the wing, known as the lift-to-drag ratio. Overall drag is also important, for instance a cargo plane will typically have a lower glide ratio than a passenger plane, but in generally these differences are a smaller effect than the wing itself.
Due to the way wings work, larger wings are more efficient than smaller ones of otherwise identical design. For this reason, large aircraft like airliners have surprisingly good glide ratios, as long as the engines are at idle (as opposed to unpowered). In addition, the shape of the wing is important in that wide wing tips generate more drag than skinny ones, due to an effect known as induced drag. For both of these reasons sailplanes use very long wings that taper almost to a point.