Oh, see, this also explains a lot! I'm still very impressed that you can write that much in your head and memorize it, but for me, the point where I can't just totally pants something with no outline in one sitting is around 4k. (That's also roughly the changeover from "getting a beta would be a waste of time" to "needs a beta or I won't post it". So maybe you don't work well with betas because you're writing things too short to need them. There aren't really any thorny plot snarls to work out in such short fics.)