Chapters are, frankly, whatever you feel comfortable defining them as. I think Les Misérables could be described as one chapter per scene (although 19th century prose tends to have a lot of writing that's hard to describe as "scenes". It's more one chapter per topic in a lot of cases.)

Writers got into the habit of writing chapters to a set length because they were publishing them as serials. The magazine or newspaper wanted a set number of words per week or per month, so they might chunk those words up into say, one chapter per week (for Sunday publication) or three or four chapters per month, (for monthlies).

When straight-to-paperback publication overtook serials though, the chapter just became a way of organizing books so people could find their place again. So you wanted something of comfortable length for reading without a break, and to allow a TOC that would fit on a page or two. Hence, the 3K to 4K standard many writers use.

But there just isn't a rule. It's one of those things you decide for your novel.
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Regards,
Eric Fretheim
Assistant Prep Leader, 2015 October NaNoWriMo Prep Challenge

"It is perfectly okay to write garbage-- as long as you edit brilliantly." ~C.J. Cherryh

“No, writing 50,000 words in a month is normal. You are not crazy. This is not insane.” ~Teri Brown