iCloud drive taking 58 GB on my internal SSD. Why is that?
Depending on settings, for both Dropbox and iCloud the files may continue to take up space on the originating computer, and on any computer that has been "synced" to allow offline usage of the files.

For iCloud you control this behavior by the storage preferences:

Open "About This Mac" from the Apple () menu, then click Storage. There is an overview of your free space, space used by different types of files, and options for optimizing usage.