Why does Oracle indicate my table is using so little space?
Your table's segment is only 2 MB.  The rest of the data is stored in the LOB segments which are physically separate from the table segment.

SELECT column_name, segment_name
  FROM dba_lobs
 WHERE owner = 'MYUSER'
   AND table_name = 'MYTABLE'


should show you that there are two LOB segments associated with the table.  You can then query dba_segments to get the size of those segments 

SELECT sum(bytes)/1024/1024 mb
  FROM dba_segments
 WHERE owner = 'MYUSER'
   AND segment_name IN (SELECT segment_name
                          FROM dba_lobs
                         WHERE owner = 'MYUSER'
                           AND table_name = 'MYTABLE')