There might be a good chance that might have used the EWW Image Optimization plugin on your child sites years ago to optimize site images. It also means that older versions of the EWWW image optimization stored image optimization records in the postmeta database table.
The EWWW image optimization plugin now uses two custom database tables to store images and queues in. In order to be able to remove postmeta records easily created by the EWWW IO plugin, you will need to install the plugin. EWWW IO plugin has a tool to be able to migrate older records from the postmeta database table into the newer custom database tables. With the plugin installed go to;
Tools > EWWW Image Optimizer
Then click on the Migrate Optimization Records button and wait for the progress of the older records to be migrated.
Now that the postmeta records have been migrated to the custom database tables you can now go ahead and deactivate and delete the plugin. The EWWW IO plugin will leave two database tables behind;
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You can use a plugin like WP Optimize to then search for the
ewww database tables and then select the two database tables to remove. It is also possible to use phpMyAdmin or Adminer to then drop both of the two EWWW IO database tables.
WP Optimize > Database > Tables
Being able to remove older legacy postmeta records will cut out of database bloat and help keep your database down in size.