Please note several imortant things regarding this values:
1) Virtual Journal Total Mailbox Item Size Limit - corresponds to archived data size in EV Journal store which means that once extracted to O365 resulting size will be bigger. Standart recommendation is to set this value to 75000 Mb in order to achieve 100 Gb resulting mailbox.
2) This setting will take effect only during item collecting and routing which means that it cannot be changed once ingestion has started without mapping deletion.
If you hit mailbox limit you have 2 options:
1 - Request MS to expand default mailbox limit https://support.office.com/en-us/article/enable-unlimited-archiving-in-office-365-admin-help-e2a789f2-9962-4960-9fd4-a00aa063559e
2- Re-create mapping and restart migration in order new settings to be effective.