Expdp with metadata_only gives way too large dump. Why?
What version are you running? I'm not able to reproduce your results on 11.2.0.4. Here's my schema:

16:36:09 system@DB&gt; select owner, sum(bytes)/1024/1024/1024 GB from dba_segments where owner = 'USER' group by owner;

OWNER                                  GB
------------------------------ ----------
USER                           1156.41901


Here's the export statement: expdp system/x schemas=user directory=DATA_PUMP_DIR content=metadata_only logfile=test_meta.log

Here's the sizing on the dump file:

oracle@db01:(db1) ~]$ ls -ltrh /u01/exports/db/expdat.dmp
-rw-r----- 1 oracle oinstall 25M Nov 20 16:40 /u01/exports/db/expdat.dmp


Make sure you're using SYSTEM to do exports, as sysdba privs are not recommended for running data pump (see Invoking Data Pump Export. I wonder if that might be the problem.