CHGPF mylib/myworkfile REUSEDLT(*YES)</pre>
The file appears to be continually adding new records at the end of the file rather than reusing space that has been made available by previously deleted records.
The previously deleted records can have their space returned by reorganizing the file.
It probably doesn’t actually matter which one runs first. Once both commands run, the problem shouldn’t come back unless <mylib/myworkfile> is recreated without setting the REUSEDLT(*YES) attribute appropriately or changed again with CHGPF REUSEDLT(*NO).