Sun made a number of workstations running flavors of Unix (SunOS and later Solaris, which were BSD and Sys/V based respectively), as well as servers including some big(ish) iron and later blades, but nothing that would be really be considered a "mainframe".
On the contrary, Sun's motto for a while was "the network is the computer" - so you'd have services like NFS, YP/NIS, DNS, telnet, rsh, ftp, etc... split across more reasonably priced servers and workstations as opposed to one big massive mainframe with dumb terminals, though dedicated X terminals logging into Sun servers was common.
Overall, very much horizontal scaling. The larger capacity servers would be for running large DBs and maybe web servers with "huge" (for the time) datasets or larger numbers of clients. It was common back then to log into a workstation on the console, then log into the back of other unused workstations to spread your work.
Edit: bonus lore: "Sun" was in fact an acronym, perhaps a backronym - "Stanford University Network". The logo was the characters S, U and N rotated about a central point - a pattern somewhat reminiscent of a certain "naughty windmill".