There are several different types of glass fibre - the one we usually use is called E-Glass - a description going to back to the early days when glass was used primarily for E-electrical insulation. This is not the strongest glass available, that is given by a S-Glass not so common here but you may see it advertised for sale in the States. A-Glass is a third type of glass, the lowest strength/quality often used in chopped strand matt for simple filling. Since the fibres are not woven and therefore have no coherent direction, chopped strand adds only a little strength to resin matrixes. Even so it is still a useful material for filling although a little heavy. In fact, glass is the heaviest of reinforcement materials with a density (S.G. if you prefer) of ~2.5g/cm³.