The Tags column is the first column in all my libraries. Why do my unread items have a large blue dot in some cases, and a tiny blue dot in others?
The larger blue dot is the default unread abbreviation for libraries created on macOS 10.12 and later. Other abbreviations such as ⚑ for flagged were also updated at that time. You can edit the abbreviations in the Tags inspector window or use the Tag Abbreviation Style script to update all built-in abbreviations for the current library.