In ArcGIS Online, the size of the hosted feature layer remains the same after the fields with records are added or deleted.

Workaround

1. Add the fields from ArcMap or ArcGIS Pro.
2. Overwrite the existing service or publish a new one. 

Note: Sometimes the item size does not update even after applying a workaround if there are no features in the feature layer. Add a new feature to trigger the item size update. It may take a few hours to update the size. 

Steps to Reproduce