When using graduated symbols for a point layer, I am able to use an expression to set the size:
For example square root:
However, when I use a pie chart as symbology, and want to change the sizes, I can only choose between "fixed size", "sum" or "field".
Due to workflow reasons the database where the data is located can't be changed, so I can't add a new field, and using an existing field creates too extreme size differences. It would be nice to be able to use my own expression as with the graduated symbols to set the size!