"Can I marry your daughter?" "Sorry, no" or "Sorry, no, she's already married" isn't less polite inherently than a 30 page screed about how you would rather just give her a shotgun to kill herself instead of torturing her like that, etc.
In fact, my shorter replies are usually my most polite. I think there is a point where it gets too short (single word answers, like some of pg's answers on hn, might be below the lower bound).
Counter-examples aren't a very good denial of correlation though. Sure, you can come up with examples of long replies and short replies where the longer reply is less polite, but that doesn't say anything about the general trend.