混淆数据库大小并获得内存错误 - wordpress

I am facing strange issue. In Cpanel it shows database size : 300 MB , Then I go to database and have ran query to get each table size , now when do SUM of all table sizes it comes to 52 MB only ,so what is issue here?

I am getting frequently below error :

Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 72 bytes) in /home2/eps/public_html/wp-includes/wp-db.php on line 1938

I have tried all possible to solve this error but still not success.

Also can anyone please tell me that is Database size == all table size sum of that database ?? or it can be diff.?

Also please give me way to remove above error for permanent

Our memory plugin showing following status :

Usage Sample: 39.37MB (15%)

Peak Usage: 39.47MB (15%)

All-Time: 255.94MB on 12/10/14 @ 3:40 pm (shutdown)

Load Averages: 0.09 0.02 0.01

WP Memory Limit: 256MB

PHP Memory Limit: 256M

Memory Usage : 85.17 MB avg.

Have just run P3 - scan wordpress plugin and got following audit, hope this help supporter to help me :