What makes you think this hasn't happened already/doesn't happen continuously?
I remember a point where pages were recommended to stay below ~40k total. With pages easily averaging over 1.2M or so more recently, it would just be insane to think whatever that cutoff is doesn't get periodically adjusted.
The real "problem" you're bringing up is that no engine is likely ever going to publicize a specific number. It wouldn't be very useful anyway, as that number alone doesn't tell you much of anything except in very simple, specific cases. A large page isn't necessarily even top-heavy, for that matter.
It's also likely–this is speculative, but plausible–that the content being "early" in the document is much less of an issue than it may have been in the past. There are methods of determining what the main content of a document is(think of how Instapaper works) that it would be trivial to apply here with Google's resources.