Looking at West Kennet it seems the difference between internal and external length is all about a tribe/family or whatever proclaiming its importance. I think the interiors were built large enough for their needs, long hard work with all that corbelling, then the covering was applied and just heaped up for further along. Bearing in mind the hilltop position of West Kennet and also that a new mound of freshly exposed chalk would have been pristine in its whiteness, the resulting long barrow would have said to everyone "Look at us and how magnificent our barrow is!"
Perhaps status, perhaps a warning to other tribes, prehaps a boundry marker or perhaps a sign to whatever higher beings they sought to communicate with.