Attachment metadata: Store the filesize within each "additional size" attachment entry.

Stores the attachment’s filesize within each “virtual sizes” in the attachment metadata. Keeps us in line with how WP core will be handling this same behavior in WP 6.0+: https://core.trac.wordpress.org/ticket/49412

https://github.com/Automattic/vip-go-mu-plugins/pull/2991