Turns out there's a very good reason Just Cause 3 is full of cars and planes and choppers and grappling hooks and parachutes: its island is enormous.
Reader Conor -- who suffers through this kind of thing for your enjoyment -- plotted the farthest distance across land in Just Cause 3's map and started strolling (and occasionally swimming).
It took him an astonishing 8 hours, 40 minutes to do it. For reference, it took him 56 minutes to traverse Fallout 4, and under five hours to get across five Grand Theft Auto games. I know movement speed and player size are important factors in that beyond just raw map size, but still, eight hours. Man.