What makes Terra less of a protagonist compared to other FF protagonists?

[Peter1986]

According to SquareSoft, FF6 isn't supposed to have any actual protagonist, and the characters are intended as an "ensemble cast" - and sure, if SquareSoft themselves say this, then that's the way it is.
But the way I personally see it, there are several clues in FF6 that point out that Terra should count as the main protagonist, or at the very least one of the central main characters;
she is the one who brings hope to the human race and is asked to join the Returners, she is the one who is being chased by Kefka, she is the one who has a flashback of her own birth in the Esper World, she is the one who is required when you visit Sealed Gate, and she is the only character who actually appears after the final battle regardless of whether you recruit her or not.
There are also other minor clues, like for example the fact that you start off as her and are introduced to her, and also that her own theme is playing on the world map of WoB.
So the way I see it, the story is most definitely focused a lot on her.
So what makes Terra so different from other FF protagonists?

Think about FF Tactics, for example;
it seems like that game is about both Ramza and Delita, yet Ramza is considered the "actual" protagonist.
Also, the protagonist in FF9 is apparently supposed to be Zidane, but isn't that game just as much about Vivi and Garnet?
And what about FF10?
The official protagonist of that game is Tidus, but isn't that game also about Yuna and her life as a summoner?
What exactly is it that makes Ramza, Zidane and Tidus count more as "protagonists" than Terra?

I would have been more ready to accept that FF6 doesn't have a main protagonist if the characters had more equal roles, like the characters in Chrono Trigger for example, but there is a significant difference between characters like Terra, Locke and Celes as opposed to less prominent people like Gau, Strago and Relm.
So if FF6 actually has an ensemble cast, it is certainly a very uneven one.

[Wolf Kanno]

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.

[Sephiroth]

Terra is like Aang from Avatar. She sort of is the protagonist but the creators tried to give the others actual relevance.

[Midgar Mist]

Okay. First time I heard the term deuteragonist.

I would disagree with Kefka being the protagonist as I see him squarely as the antagonist.

I see Terra and Celes as joint protagonists. The spotlight is on both of them as the leading mages and in the World of Ruin, Celes is the character you begin with, and those scenes she has are so powerful that I give her equal billing with Terra. The weird thing is that Terra becomes optional here, which stops her from being the sole protagonist.

I see Zidane as a full protagonist as he is as integral to the plot as Cloud or Squall are in their stories.

Then, you have Tidus as the lead in FF10 and Yuna goes from whatever that word Kanno posted that I can't be bothered to look and check, to the protagonist of 10-2.