Would it strike you as odd if most chapters in a thesis were 12-13,000 words long, and one other was 18,000-19,00?

There is just no way this chapter can be split and at this late stage in the game (3 mths to submission) I really don't want to be thinking up a new structure....

Are you chapters balanced or inbalanced?

I don't think that's too big a disparity. If your chapters were mainly 10,000 words and one was 20,000 or more I'd be concerned. But that sounds ok.

Most of mine come in at about 10,000 but a couple are either shorter (7,000 or so), or longer (15,000), so I've agonised over this question too!

I agree with Bilbo, besides, it's natural for some sections to require more words than others, as long as the writing is succinct I wouldn't worry at all. (then again, I am a science PhD, so you may want to discount anything I say!)

Thanks for your replies, I will stop stressing and finish writing! Though of course the sup might have different ideas about the baance of the structure! heh