Why are different-length chapters a problem?

I’m in the process of editng my nonfiction book. I am just starting chapter 6, which is at least 50% shorter than the two chapters that came before it. Do I need to stuff extra stuff into the chapter to make it similar in length or is it okay if it’s significantly shorter? I’ve heard Thomas say that unequal length chapters is a problem, but maybe that’s only for fiction? Thanks.

Ken