Why the mailbox quota is migrated incorrectly to a server with Plesk Web Admin edition? All migrated mailboxes have 100 MB quota.
It is expected behavior because there is no service plan management in the Web Admin edition and all domains are migrated to the default service plan.
More information about Plesk editions can be found in the Administrator's Guide.
To learn how to change mailbox size on per-domain basis in Web Admin edition, please check the corresponding knowledgde base article.