Why is OST so Big - Reduce OST File Size

You might be wondering why your computer is limping to process a large OST file while it can smoothly run a 100 GB game at a full HD resolution. It is because a Large OST file or an oversized OST file might require a lot of bandwidth to sync with the server that holds your OST file. So, if you ever encounter any error related to an oversized OST file you find all the information on how to fix it in this article.

Why is My OST File so Big?

OST file stores all the Outlook data that is accumulated from the point you created your account. Additionally, it also stores all the attachments, contacts. Generally, it might not sound like a lot but, in a long run slow and steadyly, OST file grows size.

Why are Large OST files are Such a Huge Problem?

Firstly, an OST or Offline storage table file works on the principle of synchronization. That means you can work on your Outlook even while you are offline and later sync with the server when the internet connection is available.

Since the working of an OST file is entirely dependent on synchronization, the larger the OST file, the longer it takes to sync with the online server and result in Outlook sync errors.

It doesn’t stop there, Outlook keeps performing regular data integrity checks so once again, larger the OST longer it takes to check, sometimes it might just get stuck in verifying the integrity otherwise known as Outlook stuck at verifying data integrity error. This is why a Large OST file which sounds mild can become highly complicated to handle.

How Big can an OST File Be?

Version OST File Size Limit
Outlook 2007, 2003 20 GB
Outlook 2010, 2013, 2016 50GB


Note: Whenever you are fixing any error, a smart step you can take is to archive the OST file contents. Because, when you are working to fix any Outlook related errors, might damage the OST file. The instant and effortless way to backup the OST file is by using Remo Outlook Backup and Migrate, a one-stop tool for all your Outlook related issues. With the help of Remo, you can Backup, Restore, and even Migrate the OST file.

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Fortunately, you do not need to stress yourself on how to fix the oversized OST file, Outlook has an inbuilt utility that will help you on reducing the size of the OST file which is known as the “compact now” option.

How to Reduce the Size of the OST file?

Before compacting the OST file, The practice of creating a backup of your OST file is considered the best practice. One simple method is to recover OST to PST file and save that PST file in a safe location. You can also instantly convert an OST file to PST by using Remo OST to PST converter.

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Coming to compacting the OST file, go to Files tab > Account Settings > Data File

how to compact large ost file

Double click on the data file which opens Outlook Data file settings dialogue box with the option Compact Now. Click on compact Now to reduce the size of the OST file.

Why is My Mailbox Larger than OST file?

Mailbox is larger than OST files due to inconsistencies in the calculation. In the 2007 and older Outlook versions there were inconsistencies in size calculations and file structure which made Mailbox 20% to 30% larger in size than corresponding OST files. In the case of 2010 and the latest versions, Outlook size calculation is more accounted for, the latest Outlook uses compression technology which auto compresses text. Which again made the OST file smaller than the corresponding mailbox.

Using Clean up Option to Avoid Large OST files.

Outlook also has an option that will allow you to clean all the unnecessary files from the trash in a single click. Since, the OST file also stores the deleted items, not cleaning the deleted items pile up and result in a large OST file.

To use the Cleanup option go to Files tab > Cleanup Option > Empty deleted items folder to clear all the deleted items on a single click.

using cleanup on outlook mailbox

You can also go to Cleanup options > Mailbox cleanup wizard which is a dedicated wizard that has all the options that will help in managing redundant data existing in your mailbox.

However, these are temporary remedies, they cannot stop the large OST error to repeat. Fortunately, there are a few adjustments you can implement on Outlook that will avoid the same error happening again in the future.

Does Archiving Reduce OST Size?

Yes, regularly archiving the OST or Outlook data file will create a backup relatively reduces the OST file size on the server. Which is also going to regulate the OST file size from reaching the limit.

How to Use Auto Archive to Avoid Oversized OST file

The auto-archive is a comprehensive tool that can be really handy to extensive Outlook users. It will streamline the regular archiving without any intervention. The following are the instructions on how to use archives.

  • You can find an auto-archive wizard in the folder tab as mentioned in the screenshot
  • Open the auto-archive you will find
    • Do not archive items in these folders
    • Archive folders using default settings
    • Archive folders using these settings

using auto archive


  • Check the option “Archive folders using default settings” which will schedule the archiving every 14 days
  • By default items maturing the mentioned age will be archived every 14 days
Items Age of the Item
Inbox, Drafts, Calendar, Tasks, Notes and Journals 6 months
Sent and deleted items 2 months
Outbox 3 Months


Archived file will be saved in PST format in the following location : C:\Users\username\Documents\Outlook Files\archive.pst

This will remove all the clutter from your OST file an efficient way to avoid oversized OST files repeating again.

Why is OST so Big - Reduce OST File Size was last modified: December 1st, 2020 by Tony Landry

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