Why are barrier island lagoons usually long and narrow?

Barrier island lagoons are typically long and narrow because they are formed by barrier islands that generally parallel the shoreline. These barrier islands are constructed by waves and longshore currents in areas where the coastal slope is relatively flat.12

Because these protective barriers extend along the coast, the resulting body of water trapped between the barrier and the mainland naturally takes on an elongated, narrow shape. While these lagoons are generally long and narrow, their configuration can be altered by internal wave action, which may create cuspate spits that potentially divide the water body into smaller, more circular or oval sections.314