https://drive.google.com/file/d/1c6Z6pK6ljWhiCU8ESoPiqV9pyeYRWL7K/view?usp=sharing

As you can see, there is no "disk size" option when cloning from a "Template default"
(backend clone from DataSource). This option exists in PVC Clone.

I believe it is rare for users to be satisfied with the default golden image source disk sizes.
Usually the 30Gi or so don't cut it for real VMs