I like the picture of nickles and dimes on a card. In both cases, all the rounds are exactly the same size, so the amount of wood space is exactly 3.14/4 leaving 21.5% air. (Area of a circle versus area of a square with the same width) This ratio is NOT affected by the size of the round - if all the rounds are the same diameter the air space is always 21.5%.
As soon as you start to mix sizes, the percentage of air decreases as the small rounds fit into the gaps between the large rounds. The more variability, the more nesting and the less air space.
Split wood which is "tightly stacked" has even less air space - given that the split pieces can be rotated such that they nest together.
You can get much tighter stacking (more wood, less air) if you have a good mix of large and small sized pieces and a good stacker.