Allocating land to a nation requires more than just drawing borders on a map. When the territory across the Jordan River was assigned to the tribe of Reuben, the process was handled with extreme precision rather than broad strokes.
The primary approach among commentators is that Moses did not simply hand over a massive block of land to the tribe as a whole, leaving it up to them to sort out the internal borders. There was no expectation that the tribe would divide the land among themselves. Instead, Moses took personal responsibility for the fine details. He directly managed the distribution, ensuring that a specific, clearly defined plot of land was allocated to each individual family within the tribe.