I've seen this occur a couple of times now, at first I thought it was a glitch or maybe I mis-clicked. When I start a New Game, it displays the size from the previous game (Excessive) so I don't change it. I check the other settings and eventually click 'Start' to generate the galaxy. However, sometimes I quickly notice it generated a Tiny galaxy.
I go to start a new game, and indeed the size setting defaults to 'Tiny' on the second attempt to generate.
I'm not entirely sure what precipitates this. I do NOT have any mods loaded. Has this occurred to anyone else?