MA will give you choices and you can deselect certain types: Settings, user data, apps ... You can interrupt the process if the choices don't suit you. But having chosen a smaller internal drive than your older drive is going to be problematic.
It's not a matter of reverting to hard drives, that's the choice you made for yourself. If you want to keep all the data and there is no room on your internal disk, you have no choice other than external storage: HDDs, SSDs or cloud storage. iCloud Drive is not a long term autonomous storage solution, it's an interactive service.
You can always use an external HDD or SSD as intermediary to copy and paste user data using Finder. That's generally where the large amounts of data are.