Autogenous and Semi-Autogenous Grinding (AG and SAG) mills are the most versatile comminution devices for hard rock grinding. Selecting the correct AG or SAG mill size (installed power, mill dimensions, and drive selection) for the grinding circuit duty is a crucial step in flow sheet development, as the comminution circuit typically accounts for a large share of a mineral processing plant’s capital cost. Selecting the appropriate AG or SAG mill for comminution duty primarily depends on ore characteristics, feed size distribution, and product requirements (downstream processes). Over the years, several papers have discussed how structurally altered orebodies, such as Andean ore or ore from seismically active zones, influence feed-size distribution, ore characteristics, and measured AG and SAG specific energy. These ores typically appear very competent in standard tests but show lower measured SAG mill specific energy; therefore, they do not align with the general understanding of the relationship between ore competency and SAG mill specific energy models. This paper discusses factors affecting SAG mill-specific energy calculations that standard ore characterisation methods do not capture, and the considerations needed to improve SAG mill sizing methods.