Does the "Storage Used" number for the site collection inside the sharepoint central admin give accurate number

Question

2 day ago I did a migration for our sharepoint on-premises team site collection 2013 (which contain 69 sub-sites) to sharepoint online, using a third party tool named sharegate. and during the migration i were observing the "Storage Used" info for the site collection inside the sharepoint central admin. and i have noted these points:-

1. Once the migration started i can see that the "Storage Used" number will start increasing, which is logical.
2. But at the middle of the migration the number reached 15.7 (GB) , then suddenly it got reduced to 10.3 (GB) and start increasing again till it reached 17.43 (GB) when the migration ended, as follow:-

3. then after one day, where the site is still not accessible by end users, i did the following actions on the online site collection:-

   • I set some empty sub-sites to not appear on search , by setting their “Search and offline availability” to “No”.

   • I also saved a sub-site as template

   • I Created a new sub-site based on the template.
   • Inside a sub-site i removed around 4 small-sized pdf files (total size of each is less than 300 KB). and i remove them from recycle bin.
   • i created a list workflow using sharepoint designer and i publish it.

But later on today i checked the "Storage Used" for curiosity, and i have noted that the storage got reduced from 17.43 (GB) to 17.14 (GB) !!, as follow:-

I checked the recycle bin and i did not find any thing deleted (again the site collection is still not accessible by end users "they do not have permissions").

So i have these 2 questions:-

1. what is the technical reason that during the migration the "Storage Used" reached 15.7 (GB) , then suddenly it got reduced to 10.3 (GB) and start increasing again?? as the migration is a process of uploading data, so the "Storage used" should never got reduced during the migration, is this correct?

2. After one day of completing the migration, Why my site collection "Storage Used" got reduced from 17.43 (GB) to 17.14 (GB), although nothing were deleted, as mentioned above?

Thanks

Answer 1

This is odd, but you can look at the audit logs if someone didn'T deleted some files or uploaded after it in Security & Compliance center:

These kind of migrations used Microsoft Storage for fast uploading, so the changing of size during migraiton is interesting but I think here is not bad.

If you need more information I recommend you to contact ShareGate support for more information about migration process.