Since this is an external table (EXTERNAL_TABLE), Hive will not keep any stats on the table since it is assumed that another application is changing the underlying data at will. Why keep stats if we can't trust that the data will be the same in another 5 minutes? For a managed (non-external) table, data is manipulated through Hive SQL statements (LOAD DATA, INSERT, etc.) so the Hive system will know about any changes to the underlying data and can update the stats accordingly.
Using the HDFS utilities to check the directory file sizes will give you the most accurate answer.