I'd like to know the same information for the same reason: how many AGO credits could this feature class eat up? Below is a screenshot of a FGDB feature class from ArcCatalog. Is this available in Pro? If not, I'll add it to the ArcGIS Ideas.
I'm not an expert on databases, but I understand that many RDBMS's will pre-allocate space for database contents, as Vince mentioned above. Then as features/rows are updated or deleted, the size of the data doesn't really shrink until something like 'vacuum' in Postgres is run to clean out 'dead' rows. So I can appreciate why that would be a difficult question to answer for enterprise (SDE) geodatabases. But it would be nice to have this functionality for FGDB's.