The disk image is 497GB smaller than the target device

An ISO file is basically a disk image of an optical disk. Disk images are usually meant to be restored/extracted to a disk or partition, replacing its previous contents.
DON’T PROCEED WITH RESTORING, THIS IS NOT YOUR CASE!

According to the size of your ISO file (3.1 GB), it might be a DVD image, so it could work when burnt to a DVD.

However, an ISO file can also be used as an archive. If you want to read files contained in an ISO file, you can mount it. This can be done using GUI or the following command:

sudo mount -o loop path/to/your/iso/file/YOUR_ISO_FILE.iso /mnt

(If you’re using /mnt for other mount(s), select another location.)

To unmount it later, issue:

sudo umount /mnt

You should also be able to browse and extract files from an ISO file using common archive managers, just like if it was a ZIP file, for example.

As @Melebius already said, ISO is a disk image format, and if you want to access the files, there is no need to restore it. Restoring to another disk will erase all the content on the chosen drive or at least destroy the drives file index.

Instead, you can simply mount the image. This is also the default action on a fresh install of Ubuntu 19.04. If it's not set as default, you should still be able to right click on the file and choose "Open with Disk Image Mounter".