Take some physical geography into account. For example, the western border of the state of Mississippi is made by the Mississippi River. That is the primary deal about boundaries.
However, humans can sometimes take other factors to far greater expanses. Maybe its because there is this large population center that a state would take advantage of to become stronger in the nation they lie in, so imagine a weird AH where Nevada has control of Southern California because they could use the population centers there.
Sometimes, linguistic factors make up the borders. For example, the subdivisions of Bosnia and Herzegovina makes it where some Serbs are in the northeast, Herzegovinans are in the south, and in the middle of all of that, Bosnians are around the geographic center of the country.