Last modified: 2011-10-26 16:11:02 UTC
The edit api should accept multipart/form-data rather than just application/x-www-form-urlencoded. The encoding of large articles can needlessly double the post, while also requiring additional encoding/decoding processing.
Have you tried to see if it supports multipart/form-data now? This seems to be a PHP issue to me, not a MediaWiki issue per se.
I just tried and it works! Will update API:Edit that large text files should be done via multipart as opposed to urlencoded.