Why does the Mississippi River become much larger after joining the Ohio River at Cairo, Illinois?

The Mississippi River becomes significantly larger after joining the Ohio River at Cairo, Illinois, because the Ohio is actually the larger of the two rivers at their confluence.1

As a result of this merger, the volume of the Mississippi River swells to more than twice the size it was above the junction. This confluence marks the transition to the lower Mississippi, where the river attains its full grandeur, often stretching 1.5 miles (2.4 km) from bank to bank. While the Mississippi is the main artery, the hydrology of the lower river is heavily influenced by the Ohio, which can provide more than three-fifths of the water monitored at downstream points like Vicksburg, Mississippi.12