Why in cpanel database size is showing 0.00MB while when i take backup of database then it is 1.2 MB?

Why in cpanel database size is showing 0.00MB while when i take backup of database then it is 1.2 MB?

Your download includes your "information_schema" database too, I hope you acknowledge that, however I would not seem to think it would weigh about 10 megabytes of data, so the only other issue I would see as a concern is you database is not mapped so try repairing it, or creating a new database and uploading the contents to it.
I believe CPanel runs a series of tasks which update counters such as database size, rather than calculating it on-the-fly. If it's not up-to-date, either the task isn't running or isn't running often enough.