They intentionally overfill them so new buyers don't feel like they got a half-empty bag of potato chips.
I find my correct amount of fill by removing a cup or so of the sand, and placing the bag tits-down on the back of my dining room chair.
I want the fill to still separate the rifle from the back of the chair, but not much more than that. If you flop it over the edge of a 2x4 it should have plenty of room between the board and the rifle.
As you use the bag it will stretch a little, and you will get more particular about how much fill you like in it. Just adjust accordingly.