Thank you for your help.
I have more than 1TB available so that should not be an issue.
I have noticed that the images that are only smart previews in LR CC and not sinced are scans from film that have been imported over the years into LR... Like all the digital images the film scans originals reside in folders on the same hard drive which is always online and accessible by LR Classic, so I don't think there is a difference between how the film scans and digital files are entered in LR Classic... But they are not synced!
Alex
Lightroom Classic only syncs images that are in collections that are set to Sync with the cloud so I would guess those images are simply not in a collection that is being synced. For those images it will only sync smart previews. If you migrate your catalog to Lightroom CC those images that were not synced before will all start syncing in Lightroom Classic too as soon as they are uploaded.