In the powder metallurgy method of composite material production it has been noted that the size ratio, between the constituent powders used in Aluminium-Silicon Carbide non-continuously reinforced metal matrix composites (MMCs), has a bearing on the final mechanical strength of the material. The exact mechanism is not understood. This work concentrates on how the difference in size ratios between the powders affects the packing fraction when the powders are poured into a die, before compaction, and how this is related to the final mechanical strength of the material. The packing of binary powders was firstly treated mathematically. An analytical model to predict the packing fraction of the powder was then tested to ensure its accuracy when used with metal powders of this type. The model was found to predict the packing fraction of the powders used to manufacture these materials within a range of 8%. Powder mixtures of various size ratios were then used to prepare material samples and the changes in packing fraction compared to the changes in the mechanical properties of the material. Comparison of the changes of mechanical properties of the materials as size ratio is changed, with the way that packing fraction changes with packing fractions shows that they alter in a similar manner. It is concluded that the change in powder size ratio leads to a change in the packing fraction of the powders before compaction, which leads to a change in the mechanical strength. However it is shown that other factors such as powder size and sintering times and temperatures also have an important part to play. It is also shown that the powders of differing sizes must be present in sufficient quantities to achieve high packing fractions. Therefore it may be easier, and cheaper, to successfully sinter material made from mixtures of powders where significant portions of powders of different sizes are used.