Nokia is rumored to be preparing a Quad-Core, 5.2" display screen Lumia 825. Like I said it's a rumor or a well calculated guesstimate.
After all, Microsoft will be releasing an update (GDR3) for Windows Phone 8 later this year and it is meant to provide support for 1080p resolution, allowing for larger screen size and display sharpness.I
It will also support multi-core processors.
Nokia has already released a 4.7inch IPS display phone, the Lumia 625. So it is quite possible they are ready to manufacture something bigger and stronger.