Well, first of all, apologies to the Bard and everyone else for that pun. It's actually a serious question I'd like to put to fellow sourdough breadheads.
Sometimes I end up with a tighter crumb than I like with breads I've done many times with identical flours and using the same process, with the same baking times in the same oven. At their best these breads have nice, open spongy crumbs. I know there are multiple factors at play here (proofing, ambient temp, shaping etc), but I'm so far unable to confidently pin down this tight crumb phenomenon to anything in particular.