Why PHPMyAdmin Shows Incorrect Database Size
It is common for phpMyAdmin to show a significantly different size than your file system or hosting panel. This often causes confusion when you delete large amounts of data but see no change in your disk usage.
The Core Discrepancy
phpMyAdmin measures Data Length (actual content), while the disk measures File Size (allocated container space).
Reclaimed Space (The "Hollow" File)
When data is deleted from a MySQL database, the space on the disk is not immediately released back to the operating system. Instead, MySQL marks this space as "claimed but unused."
Think of it like a bookshelf: if you remove a book, the shelf (file size) doesn't get smaller; it just has an empty slot. When new data is added later, MySQL prioritizes filling these empty slots before requesting new space from the disk.
The InnoDB Factor
This behavior is a specific feature of the default storage engine, InnoDB. Unlike MyISAM, InnoDB is designed for performance and reliability.
- When you drop a table or delete rows, InnoDB keeps the disk space reserved.
- This reserved space is tied to that specific table (or tablespace) for future use.
- phpMyAdmin calculates size based on "Active Rows," ignoring this empty overhead.
🛠️ How to Fix It
To force the database to release this "overhead" space back to the operating system, you need to rebuild the table.
You can do this by running the OPTIMIZE command.
OPTIMIZE TABLE your_table_name;Note: Alternatively, performing a "Dump (Export) and Restore" of the database will also completely reset the file size to match the actual data.