When symbolizing data with a Pie Chart layer symbology option, it is possible to vary the size of pie charts on a feature by feature basis. There are two options for varying size. The first option will vary the size of the symbols based on the sum of the value fields in the pie. The second option lets you specify a "size" field whose data values are then used to determine pie size.
The pies are sized proportionally by area using either option. The feature with the smallest data value is assigned the minimum pie size. This size is user specified in the 'Pie Size' dialog box; data value is either the sum of the value fields for that feature, or value of the size field for that feature. For each subsequent feature, the difference in data value between that feature and the feature with the minimum data value is calculated. The pie size is then calculated based on the ratio between this difference and the size of the pie for the feature with the minimum data value.
When specifying a symbol size, the specification is for the size of the symbol that appears on a printed map at the map scale that is seen in ArcMap’s layout view. This is unless the map has a reference scale. In this case, the symbol size is being specified as if the map were being printed at the reference scale. If the map is printed at a different scale, then reference scaling scales the symbol larger or smaller. In general, it is good practice to specify a reference scale at the same scale at which there is intent to print the map.