Thanks to cantab314 for the inspiration!

Many wonders “if each Final Fantasy game takes place on the same world, why are the worlds different”? Wouldn’t the geography change each time?

5 and 6 both show ways the world’s geography can be significantly changed much quicker than natural geology would allow. Extrapolating from this, all Final Fantasy games actually take place on the same planet, with catastrophic reshapings occurring between the games.

Notice that both Final Fantasy V and VI show significant changes in the topography.

In Final Fantasy V, the world was fragmented but then brought back together again:

The Merged World is how the world originally was before its crystals were separated 1,000 years ago, thus splitting the world into two. This was done to seal the power of the Void in the Interdimensional Rift between the worlds, so none could harness its might for evil. The warlock Exdeath succeeded in claiming the crystals and shattering them, thus pulling the two worlds back into one, and in doing so, gaining access to the power of the Void.

In Final Fantasy VI, the world transformed through violent means. For the World of Balance:

The world was created by the Warring Triad in ancient times, and they were responsible for keeping the balance. During the War of the Triad their power became unstable, and corrupted various denizens of the world, turning them into magical beings known as espers. They regretted their actions and the espers agreed to seal them up and protect them so they would not go haywire again.

And again for the World of Ruin:

The World of Ruin was created when Kefka Palazzo moved the Warring Triad out of alignment on the Floating Continent. This delicate balance was the key to sealing the World of Balance in place. When Kefka moved them out of balance, they went haywire and began destroying the world in an event that would become known as the end of the world. The result was the World of Ruin.

But how? Earthquakes.

NobleV says something very similar here:

All I know is if all the FF stories are connected, then that planet has a serious Earthquake problem.

Now, of course, I think NobleV was being facetious, but why wouldn’t they? Final Fantasy originates from Japan, where earthquakes are a constant threat. And earthquakes, if the magnitude is high enough, can reshape the land’s surface:

Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings and other rigid structures. The severity of the local effects depends on the complex combination of the earthquake magnitude, the distance from the epicenter, and the local geological and geomorphological conditions, which may amplify or reduce wave propagation. The ground-shaking is measured by ground acceleration.

Specific local geological, geomorphological, and geostructural features can induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. It is principally due to the transfer of the seismic motion from hard deep soils to soft superficial soils and to effects of seismic energy focalization owing to the typical geometrical setting of the deposits.

Ground rupture is a visible breaking and displacement of the Earth’s surface along the trace of the fault, which may be of the order of several meters in the case of major earthquakes.

With the cataclysmic changes Final Fantasy worlds go through, it’s possible the same planet look like Gaia, Spira, and Eos.