It's the number of files being referenced. It's not the page size. I'm going to assume that this is an HTML article that references a lot of other files. I think the limit is set pretty high (1k files). The limit was set to avoid having someone initiate a DOS attack through the ingestion service by creating articles with lots of little 1 byte files thus overwhelming the service. We just look at the number of files and refuse the upload if there are too many.
If you are seeing this and this isn't an HTML article that references a lot of files please call tech support and they will get your article and see if they can reproduce the problem.