It seems like gravity tends to clump at certain scales - is there some reason for this? Planets, solar systems, galaxies, maybe higher order as well. Why isn’t there more of a sort of soupy spread rather than what seems to be a fairly hierarchical order, and what sets the sizes of the scales?
I’m not sure what you are saying. My guess is that you think that all planets are about the same size, all galaxies are about the same size, etc. If that’s what you mean, I think you’re mistaken, but I don’t know how to prove it.
Because if there was a soupy spread, it had billions of years to clump up and the more it clumps up, the more gravity pull it has which gives you a snowball effect.
Quantum fluctuations during the Big Bang caused subtle variations in the density of the universe. The variations spread out during the inflationary period and the later expansion, and ultimately caused the clumpiness you see at the largest scales.
In a sense, planets are just small stars that didn’t have enough mass to undergo fusion, or where all the gas was blown away to leave only a rocky core.
There are a few physical reasons why you might see gaps in the size distribution. For instance, gas giants can’t get much bigger than Jupiter–you add more matter and the extra gravity causes them to compress more. However, eventually you will add enough mass that they will start to fusion, and then expand due to the increased temperature. So you get a size gap between the largest planet and smallest star because fusion requires crossing a certain threshold.