The six regions are not equal in size and everyone is treating them as though they are

Vice City is described as roughly twice Los Santos. The other five regions share whatever is left of a state that is about triple RDR2's area.

That does not divide into five equal pieces. The Keys are a chain, which is long and narrow. A national park on a state border is probably large. A forgotten coast is probably a strip. Ambrosia is agricultural, which in real life means enormous and mostly empty.

So the useful mental model is not six regions, it is one dense city and five very differently shaped spaces around it. Every adjacency map I have seen draws them as six similar blobs and that is almost certainly wrong.