A discrepancy in Snapshot size by Snapshot management window and by Powershell Command line

I  used the command "Get-VM | Get-Snapshots | Select VM,Created,Name,SizeMB", and the result in the size on snapshots is quite a lot of different (much smaller) than the size as shown from Snapshot management window on VM's in VSphere client. I tried a number of VM's, got the same outcome.

Which one really is true?

If Vsphere is, then what command can I use to get an accurate result?

Thank you!