IMHO, such tests results are very individual and very depend on which version of Excel they were done. Even on the same version it depends, e.g. do we work with data model or not.
Second, I don't thing that the file size shall be the main reason when you make a choice between PivotTable and formulas. Lot of other factors, as flexibility in aggregation, refreshing/recalculation, etc.
If file size really becomes critical perhaps to consider other tools like Access / SQL Database / Azure storage and perhaps use Excel as interface to them.