I think or at least thought the reverse is the case at least for they type of logs that could be run through a processor. Look at this rack and the rounds waiting to be split in the picture. There sure look like more void in the rounds. Also look at how the vertical side in the splits matches the vertical end board versus how rounds would both only touch the end at one spot and perhaps not go all the way due to fitting between the rounds below.
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Otherwise your post I agree with.
Around here it seems the firewood guys that are worth using more than once have processors and load a dumping type truck loose. Even the ones that take checks don't give a receipt not sure if anyone asks. Another issue could be is having a swinging baffle in the bed which they say is so they can dump half of it only is fair or not. Obviously a vertical baffle will make more voids. I realize it is how it stacks, can't recall I have sold any firewood though I think I did put a craig list ad for as much as you could put in an 8 foot pickup no side boards.
What causes the growth in a pile after splitting/restacking is that every time you split a piece in increase the surface area a considerable extent. Yes a your picture of rounds vs stacks does seem to show less voids but really there are more voids and in total they come to more volume than the voids in the rounds stack.
Simple test you can do sitting in front of your tv. box top either round/square/oblong, whatever. a couple of nicely tapered carrots so you have varying sizes of rounds. Slice rounds and fit them one layer deep in the boxtop as tight as you can. Then dumpt those out, split in halves, quarters, whatever you want. Now fit them back in the box top. You will find you can't do it.