Okay, so I ran a direct, streamlined route through FFXIII's Gran Pulse area to compare its size to LR's Wildlands, and this is what I calculated. I started from the very edge of the map, near Vercingetorix's crystal orb in the Paddrean Archaeopolis (not sure if that's spelled right) and ran all of the way to the very tip of Oerba, near Vercingetorix's Mark crystal and where Barthandelus's ship was. Every area begins and ends exactly where the loading screen into the next begins, and that's where I stopped the stopwatch. I used the stopwatch on my phone and stopped it every time I encountered an enemy, then started it again afterwards.
Afterwards I added up everything with a calculator. Here are each area's times, then added up:
Yaschas Massif: 4 minutes, 14 seconds
Vallis Media: 2 minutes, 30 seconds
Archylte Stype: 3 minutes, 15 seconds
Mah'habara: 5 minutes, 43 seconds
Sulyya Springs: 1 minutes, 44 seconds
Taeijin's Tower: 2 minutes, 30 seconds
Oerba: 4 minutes, 26 seconds
Now, I'll describe where I started and ended in case anyone asks.
Yaschas Massif: starting from the treasure sphere behind Vercingetorix's crystal orb and running through the entire canyon, ignoring the little branches and taking one path to the Vallis Media, near the save point.
Vallis Media: starting from the entrance from Yaschas all the way to the loading screen (not including though) at the end past the yellow Cie'th Stone.
Archylte Steppe: Beginning from the passage into and crossing over the Central Expanse (ignoring all other areas) and exiting from the Northern Highplain into Mah'habara.
Sulyya Springs: after exiting Atomos, running through the springs and that rocky path to the entrance into the Pallisades
Pallisades/Taeijin's Tower: starting from the canyon-like area and curling around the Pallisades and entering Taeijin's tower but not visiting any levels in the Tower, excluding the loading time for the elevator to reach the Tower's Apex, and running to the elevator.
Oerba: beginning from the elevator and winding through the entire village and onto the bridge at the very end, the closest you can get to Cocoon near Vercingetorix's Cie'th Stone.
Again, no teleportation, no chocobos. Just a straight shot through Gran Pulse.
And it all adds up to...
22 minutes, 22 seconds.
In other words, just one continent in Lightning Returns (if the devs weren't stretching it) is two times bigger than FFXIII's largest map. Obviously this doesn't include the areas like Titan's Trials (though that wouldn't add up to much anyway, the smaller areas like Font of Namva, Western Benchland, etc.).
That is some impressive stuff right there.