SELECT TABLE_NAME, NUM_ROWS FROM all_tables WHERE owner='SCHEMANAME' ORDER BY num_rows DESC
1) ALL_TABLES is oracle system table
2) If your username IN oracle AAKASH, then here owner or shchem is aaksas in CAPITAL letters, it will show only tables belong to AAKASH oracle user.
3) there are so many properties of table, you may use * in query. NUM_ROWS is no of records in that table. And I have ordered it in descending manner by number of records
4) So when u see query result, you will find table with most no of rows, at the top.