Exchange Server mailboxes store email more efficiently than Outlook uses in a *.pst (or *.ost), as a result, when you export a mailbox to a *.pst, you can expect the pst file to be somewhat larger than the mailbox's reported size. This is due to a number of factors, but the bulk of the file size is due to how Outlook stores TNEF data with each message.
In my experience, messages in a pst are several KB larger than in a mailbox, but it can vary widely depending on the typical message format and number and type of attachments. The file size grows even more when messages are saved to the file system: messages saved to the hard drive are at least 10 KB larger, and in many cases, 40 - 50KB larger than the message size reported in Outlook.
Moral of the story: don't be shocked when messages seem to grow in size after moving them to a pst file or saving to the file system.
Hi Diane, I hope you are well. You are always the person I go to for Office advice, you are awesome!, thanks for all you do. I could do with a real quick reply if you are able to - i have a pst which i have imported to outlook and is about 20gb. it has about 1500 folders in it. i have uploaded it to a 365 mailbox via a pst import into outlook (dont ask why...). outlook itself says the mailbox is 14gb (right click on mailbox, select properties). 365 exchange admin says the mailbox is 14gb. outlook seemss to believe it has finished....i have had big problems importing this file. outlook stops uploading folders periodically, and has to be restarted, then it starts importing again. anyway, i believe it has now finished. but i would like to know...is 14gb in outlook/365 vs 20gb pst import file a likely outcome? could there really be 6gb difference? one last thing - is there a way of comparing what is in the 365 box vs what is in the pst in terms of total number of folders / total number of items so that i can check? thanks you so… Read more Âğ
You can get messages counts per folder in Outlook on the web's settings. It will be slow going if you want to check all of the folders - but you can spot check folders here and there. As for the sizes - if the pst was actively used, it could need compacted - open it in outlook and compact it then see what the size is. That will give you a better idea of file size. On a 20 GB pst, compacting could reduce it by 3-5 GB. (If you exported from another data file to create it, it won't need compacted - that would be an accurate file size.) That said, pst files are a bit larger than an exchange mailbox, by as much as 20%, based on how the messages are stored - plus there is hidden data in the folders that is not imported or exported - rules, categories, views, and custom forms are just some of the things that are stored in the data file and can affect its size. But, again, if you exported from another data file to create the pst you imported, hidden items won't apply because those things didn't export. The import… Read more Âğ
thanks diane, i really appreciate it. it was a many years old pst, not an export. i think im happy that we have everything, thanks for the hand holding! take care and thanks for your time.
i'd forgotten about pst compaction! its been too many years of exchange and ost's......i will compact a copy of the orignal pst and see what we get which will hopefully further give me confidence.
post compaction, its 15 gb, so...pretty much in the ballpark. thanks again Diane.