The initial bulk load option for Spark bulk load sends values over one by one through the shuffle. This is the similar to how the original MR bulk load worked.
How ever the MR bulk loader have more then one bulk load option. There is a second option that allows for all the Column Families, Qualifiers, and Values or a row to be combined in the map side.
This only works if the row is not super wide.
But if the row is not super wide this method of sending values through the shuffle will reduce the data and work the shuffle has to deal with.