Table files will grow when the data grows, but they may also grow when the changes (insert/update/delete) create gaps between the real contents. Run this command to tell the cases apart:
mysql> select TABLE_SCHEMA, TABLE_NAME, TABLE_TYPE, ENGINE, TABLE_ROWS, DATA_LENGTH, INDEX_LENGTH, DATA_FREE from information_schema.tables ;
(Append "where table_schema = '.....'" to restrict the output to a certain database.)
If you see big values of DATA_FREE, you have gaps. Use "OPTIMIZE TABLE" to get rid of them, see the MySQL documentation for details.