It appears to me that they just cut the size block that was needed for each space. As in, if you put a six inch block on one side and a 9 inch block on the other, then used a 3 inch block sort of in the center area, then a 9 X 3 block on the left of the 3 inch block and a 6 X 12 inch block to the right of the 3 inch block. You would make a large block with a total of 5 small blocks sewn together that would be 15 X15. Then make another large pieced block to add to it, etc. etc.
Does this help?