Republish the layer from ArcGIS Pro, when extracted from ArcGIS Online, to get the correct size of the feature layer.
Overwrite the layer from a new project in ArcGIS Pro to update the size of the feature layer.
Add a new feature to trigger the item size update, but it may take a few hours for the size to reflect the change. Note: Sometimes the item size does not update even after applying a workaround if there are no features in the feature layer.