Well for the case of VI, several of the characters have moments to be in the spotlight just as much as Terra and while Terra is certainly central to the story, her role and and screen time isn't really any longer than other characters. She doesn't even have to participate until the ending in the WoR, so she's more absentee than other heroes you've listed. She's only really prominent for say a third of the game despite her role kickstarting all of the plot points.

Ramza is the protagonist of Tactics while Delita would be the deuteragonist, which is a role just as important but slightly behind the protagonist whom the player largely explores the whole story through them.

Tidus' case is having a protagonist who is witsnessing the story of the deuteragonist (Yuna) and his story ultimately becomes more important than hers.

Zidane is a bit of the odd ball in this. IX is meant to be an ensemble cast but Zidane is often chosen to be the eyes for the player for much of it, so he basically gets the "main protagonist" moniker because he has the most screentime and is the character the player becomes the most familiar with. Lightning does the same thing in XIII despite it also being an ensemble cast.

In the case of how not all of VI's cast are given equal treatment, its largely due to Square following Hollywood rules over theater rules, in which case only several of the character need to be prominent. Locke, Celes, Edgar, Sabin, and Cyan have lots of screentime and prominent roles whereas Gau Mog, Umaro, and Gogo are largely side characters, Shadow and Setzer are more supporting character, while Strago and Relm are combined to tell one story. There was actually a pretty cool article someone linked to a few years back that argued that Kefka is the real protagonist of VI because he is the real driving force of the plot who connects all the characters and events together.