I don't know about using different size harddrives, but I cloned same size USB thumb drives and it works.
I cloned a Zorin OS installed onto a USB thumb drive on my lenovo T430 thinkpad. After I installing it, I used a free windows program called 'Active Disk Image Lite 11.0 - Freeware' to backup the USB to an image file (It can back it to a compressed image file to save space). Then a wrote that saved image onto a second USB drive (exactly same USB model & size) and it runs OK like the first USB does. So, yes you can clone Zorin install and run it. The Zorin install runs OK in all my other lenovo thinkpads (3 different models).
Just done a search and the best way is to put SSD in an enclosure:
Clonezilla is tricky - Disks creates an .img file backup but you might hit problems using that to fit on a smaller drive - another option would be to shrink you larger Linux installation to match the smaller one - but not sure if you could expand afterward.
Replace Ubuntu 18.04 with Zorin 16.1! (hopefully!)
You are doing fine there I see. But you should rather use the cog wheel for the partition(s) you want create an image for instead of the 3 vertical dots. The 3 vertical dots lets you image the total drive, while the cog wheel is for individual partitions.
As for space, I remembered that the application Disks (and also Gparted) creates an image of the total partition (instead of the present data). Which means you will have to reverse the same or bigger size partition which to restore to. Just like Dav suggests, but he used identical usb (with the same size).
Well then your first try was a better option. If the size is the same, I don't see why there should be a problem. If the drive is attached you can use the same application, and restore the image, the same way you saved it.
I'm not that familiar with windows partitioning. Never use it. I like how the ability Linux/ Gparted provides while resizing or adjusting a partition. Almost never encountered any problems. And if I did I just reformatted and started all over again, because you have the original image. So no harm or pain there. Its trial an error, once you get used to it, it'll be a very easy.
I once did try the same like you and it worked, even boot files and grub settings were copied and restored. So I was able to use data from one partition on a second drive.
Edit, after the restore you will have to run the update/upgrade because of the drivers on the second pc. Just like Aravisian suggests.