When taking config only farm backup getting insufficient disk error

Hi Guys,

I was trying to take the config only backup of the SharePoint farm. The estimated size was showing 1.58 GB. My backup drive has 69 GB free but when start the backup it gave error not enough diskspace and showing 189 GB space is required for this. 

Can anyone please explain why it is showing this error ?

I tried using CA and PowerShell both.