The size of mdf file is much bigger than its gdb file,
As all the content of mdf file is exported (or imported) to a gdb, the size drops dramatically. The size of mdf is 8GB while its corresponding gdb is 200MB!!!
Then how to “clean” the mdf file so that it gets as small as the gdb?
Read up on sp_spaceused (Transact-SQL), paying particular attention to how the categories of space are defined.
Shrinking an enterprise database (geo- or otherwise) can cause a lot of logging, disk I/O, and other non-desirable behaviors. Given the size of today's local storage options, I am not sure it is worth pursuing. If you do pursue it, make a backup before you try it.
Another thing to consider is that the geodatabase model is implemented quite a bit differently in an enterprise geodatabase versus a file geodatabase. File geodatabases may support the data models of the geodatabase, but they don't support all of the functionality like versioning.
Have you looked at the state of your geodatabase? How much data is tied up in add and delete tables? I would look at reconciling and compressing the geodatabase before shrinking the enterprise geodatabase that hosts the geodatabase.
Are there any powerful tools in Pro that can purge the mdf data?