How is Per Object Storage Usage Calculated

Data Storage is not calculated by the number of fields or content in the fields. It calculated as a number of records. Custom Object take 2KB per record. Doesn't matter i if they have 1 field or 200 long text fields populated.

Salesforce uses a simplistic method for calculating storage usage. Most standard and Custom objects are 2KB but some special objects like PersonAccount take 4KB(as its an account and contact)

So in your case. 9999 records take

 (9999*2)/1024 = 19.52MB

SRC: https://help.salesforce.com/articleView?id=000193871&type=1

What you are seeing in Data Storage is not based on the size of the field for respective object. The storage reflects the size of overall record that is stored in that object. Salesforce record size overview mentions that any record is roughly around 2KB in size (with some exceptions).

In your case the storage is approximately around 19.5MB based on 9999 records for each object.