As data gets migrated from one environment to another, mailbox size may change considerably. MigrationWiz does not "inflate" migrated content. The change in size is due to differences between the Source and Destination environments.
- One system may compress data, while another may store data as is.
- One system may store content using flat files, while another uses a rich SQL database.
- One system may perform full content indexing, while another might index only subjects.
- One system may store a single copy of an attachment appearing in multiple emails, while another may not.
- One system may store different MIME parts (for example, plain text + HTML), while another may convert content upon rendering.
In the special case of Google-based migration, you can find more details about size increases here.