It’s possible that the file you restored is not the right one being the reason why the backup is showing half the size. I recommend locating and opening the copy of the company file created before the installation of QuickBooks Desktop 2019.
Click the File menu and select Open or Restore Company.
Choose Restore a backup copy, then click Next.
Select Local Backup, then hit Next. (Screenshots attached for steps 1-3)
Search for the backup file from the Look In drop-down.
From the folder, the file name should appear as [Company Name].qbb. Select it and click Open, then Next.
Save the restored file on the location where you can find it easily.
In the File name field, enter a new file name.
Once done, try creating a backup copy of that file. It should have the same size as the one you restored. Check out the following articles for more information: