Less is more with writing samples?

[ccbaxter]

Around half of the humanities programs I'm applying to are asking for a writing sample of around 20 pages (some are 15–20, some <25). But others are asking for up to 30 pages or have no stated limit.

I'm submitting part of my master's thesis, so have extracted my introduction (lightly condensed) and my first chapter, which comes to about 18 pages total. I like what I have; it's by far my strongest chapter. My other chapters are a different story. I mean, they're not terrible by any means. My two graders liked them for the most part, but said they were a little disjointed and structurally awkward. And, honestly, I don't feel like I have the time or the energy to make real substantive edits to either of those chapters (I know, I know…). But I worry that submitting 18 pages when I'm being told I can submit up to 30 seems stupid.

Is it worth sending a longer but weaker writing sample, or a shorter but stronger one? Will 18 pages look weak when others are sending in 30?