The only thing that comes to mind is that they're prepairing for an onslaught of media like Movies (which is the only thing that would take up that much space).
The new limit means that instead of installing large resource files that need to be hosted on your own infrastructure and downloaded through your own code, you can now have Google host these files for you.
This also improves the user experience, because there is no second tier install process and the users sees the correct space requirements on the market page.
This change would probably help fix issues where the user can't download the content, but could download the app through the market.