List of steps to reproduce (step by step, including full links if applicable):
Error is reproduced by this call: https://ja.wikipedia.org/api/rest_v1/page/mobile-sections/JIS_X_0213%E6%BC%A2%E5%AD%97%E4%B8%80%E8%A6%A7%E3%81%AE1%E9%9D%A2
It is a call to [[w:ja:JIS X 0213漢字一覧の1面]], which is currently ~597k bytes in size.

What happens?:
The following is returned.

{"type":"https://mediawiki.org/wiki/HyperSwitch/errors/unknown_error","method":"get","detail":"413: https://mediawiki.org/wiki/HyperSwitch/errors/unknown_error","uri":"/ja.wikipedia.org/v1/page/html/JIS_X_0213%E6%BC%A2%E5%AD%97%E4%B8%80%E8%A6%A7%E3%81%AE1%E9%9D%A2"}

What should have happened instead?:
The error message does not point to the correct page - the type and detail fields should point to https://www.mediawiki.org/wiki/HyperSwitch/errors/too_large instead.