Mailbox sizes increase dramatically after migration to exchange 2013

Question

After I migrate mailboxes from exchange 2010 mailbox to 2013, the size increases.
For example -
On exchange 2010 I run get-mailboxstatistics: TotalItemSize : 6.706 GB
On exchange 2013: TotalItemSize : 9.374 GB
Anand_N

Answer 1

Hi Anand,
Please note that the actual space used by the mailbox database will not increase. This only refers to the attribution of space used by each individual mailbox. Actually, this happens due to the inclusion of all item properties into quota calculations, providing
a more accurate computation of space consumed by items within their mailbox. However, you can adjust mailbox size and increase mailbox quota by 40% before moving.
You can follow below cmdlet in order to increase "ProhibitSendQuota" value for all users that are not using the database quota defaults:
Get-Mailbox -ResultSize Unlimited -Filter {UseDatabaseQuotaDefaults -eq $False} |
Where {$_.ProhibitSendQuota -ne "unlimited"} | ForEach {Set-Mailbox $_ -ProhibitSendQuota "$($($_.ProhibitSendQuota).Value.ToMB() * 1.4)MB" -WhatIf}
You can have a look on this link to prevent the cause :
Mailbox migration from 2010 to 2013
Hope, this will be helpful for you.