Performing a “Combine geometries” (Alt+C) on your scene sometimes reduces and sometimes increases the file size. Why does this happen?
The natural expectation when using “Combine geometries” mostly is a general decrease in file size as the complexity of the scene structure is reduced but this is not always true.
If a scene contains a lot of referenced objects combining geometries will resolve those references. When the references are resolved the transformation values of the original geometry are applied directly to each of them which leads to an increase in file size.
The final file size also depends on the scene tree depth level at which “Combine geometries” is performed. Objects from below the specified level are combined into common bodies which allows a more effective compression.