The unit loads, e.g. plastic containers or EUR-pallets, used in an assembly plant can have a significant impact on time efficiency, and hence cost, of both the materials supply and the receiving assembly stations. Smaller unit loads can reduce the time the assemblers spend fetching parts. However, larger unit loads result in fewer moves for a given volume of materials, which implies efficient in-plant materials supply. The current paper explores how the time efficiency of in-plant materials supply is affected by the size of unit loads. Based on the case study, it is clear that the efficiency of the in-plant materials supply is not proportional to the size of the unit loads. There are fundamental differences between how large pallets, compared to smaller unit loads, are delivered, meaning that the increased delivery frequency required for smaller unit loads does not necessarily result in an increased man hour consumption.