Newer versions of the WordPress plugin All-in-One WP Migration have been re-written to refuse to work if the migration file is larger than 512MB. The plugin has been deliberately crippled to induce you to pay for an upgrade that unlocks larger file sizes.
One thing that is (in my opinion) a little sleazy is that the export function will create and download any size file you have. It’s only after you need to import that you get mugged for the upgrade.
The nice lady at https://marionblackonline.com/all-in-one-wp-migration-plugin-hack/ showed how to get the old version of the All-in-One WP Migration plugin. Version 6.77, although it has the artificial limit, can be edited to a different limit. Getting the file is slightly opaque, as the plugin page on wordpress.org does not let you go back that far, version -wise. But the file is on the servers, and downloadable.
https://downloads.wordpress.org/plugin/all-in-one-wp-migration.6.77.zip
Once you’ve installed it (by uploading it from a .zip file instead of from the WordPress Plugins store), WordPress will constantly nag you to update to the crippled version. Edit the file wp-content/plugins/all-in-one-wp-migration/all-in-one-wp-migration.php and change the Version string to an impossibly high number.
As long as you are editing files, you might as well edit wp-content/plugins/all-in-one-wp-migration/constants.php
Original file size limit:
define( 'AI1WM_MAX_FILE_SIZE', 2 << 28 );
Make it your preferred file size limit:
define( 'AI1WM_MAX_FILE_SIZE', 4294967296 ); // 4 GB file size limit