It is worrisome that Esri Support could not provide you with a clear-cut answer because from the little you have shared here, I think the answer is straightforward.
SQL Server, even the Express edition, is much more complex than file geodatabases. Additionally, you appear to be creating enterprise geodatabases (not a good idea, I am not even sure Esri supports using Express for enterprise geodatabases).
If you look at the Create a geodatabase in SQL Server—Help | ArcGIS Desktop documentation, you will see that the default size for a new SQL database containing an enterprise geodatabase is at least 625 MB:
The Create Enterprise Geodatabase tool makes it easy to set up a geodatabase because it creates the database objects and grants required privileges for you. To achieve this, the tool uses the following default settings and creates the following database objects:
- * It creates the database files (MDF and LDF) in the default SQL Server location on the server.
- * It sets the SQL Server database options READ_COMMITTED_SNAPSHOT and ALLOW_SNAPSHOT_ISOLATION to ON.
- * It uses the default database file sizes you have defined for the SQL Server model database or 500 MB for the MDF file and 125 MB for the LDF file, whichever is greater.
- * If you choose to create a geodatabase in a schema named sde, the tool creates an sde database-authenticated login in the SQL Server instance, creates an sde user in the database and maps it to the sde login, creates an sde schema in the database, and grants the sde user privileges to create a geodatabase and kill connections to the SQL Server instance.
- * It creates the geodatabase objects inside the SQL Server instance.
If you take your roughly 75 MB of file geodatabase data and add it to the new, empty geodatabase of 625 MB, you get roughly 700 MB in total size.