When creating a new dynamic VDI why is the suggested default 8GB? This is the maximum and not the actual correct? So why should not be very large so that you don't run into the limit?
8Gb is the typical minimal size for a modern OS. Even if the disk is dynamically growing, you still don't want it too large, especially because at some point you might end up over committing your storage space, and that's much worse.
With VMs it's best practice to allocate just enough space for a VM for it's OS and it's purposes.