We have run into this as well when clearing out a hosted feature layer by truncating it. (Get yourself attached to BUG-000135310 for more details, which is listed as fix, however, they seem to mean in this case that "it is working as expected.")
The workaround to reset the size, so that you are not charged credits unexpectedly, is to compress the empty feature service. And, if that doesn't do it by itself, then make an edit in the feature service to trigger the reset (i.e., add a bogus new feature, then delete that feature.) After a few more hours the size should be zero. (Our experience has been that the second step is required most, but not all time.)
One way to do all this is to go to your hosted feature layer's Item Details view, after you have deleted all the data, then:
- View the feature service URL in the lower-right of the page.
- Select the Admin interface in the top-right.
- Select Compress from the Supported operations at the bottom of the page.
- Click the "Compress data" button.
Give it a few hours for the size to go to essentially zero. If it hasn't changed, then go on to second part:
- Create a new feature in the layer.
- Delete that feature.
Wait a few more hours for the size to go to zero.
You do not have to wait though, if you don't care about seeing the size go to zero. Once you have deleted all the features, you can compress it, and immediately start editing features again, and you should see the correct size of your new content reflected after several hours.