Both of the collections I've put together so far have been linked, which made deciding what stories to include a bit easier. In fact, almost all the pieces in Cryptid Bits were written specifically for the chapbook, and the same was true for about half of the stories in Luck and Other Disasters. There was still some curation work that needed to happen—in both cases I wrote some stories that didn't end up in the final version, and it took a bit of fiddling to get the pieces I did include into the right order. But since each of those books occupies a self-contained world, with shared characters and settings, deciding which things fit into each book was pretty straightforward.
That's not the case for every short story collection. I knew that in an abstract, of course, and now I’m getting a first-hand lesson. A few months ago, I realized I have enough speculative short stories to put together a collection—way more than enough, actually, which is mostly a good problem to have. I have options and can mix-and-match things to find the exact right set of stories. But that’s also caused a bit of a delay in getting the manuscript assembled while I figure out how to even choose which ones belong together.
Read this if you like: Karen Russell, Carmen Maria Machado, quirky short fiction
tl;dr summary: Collection of (mostly) literary stories in which characters hope, for better or worse (mostly worse).
Read this if you like: mythic fantasy, punk poetry, Haruki Murakami
tl;dr summary: Combination poetry collection and story about a galaxy-traveling wizard