It's possible. But they still need to fit a giant battery in there. Even with RAM and SSD directly on the motherboard, I am struggling to think they will fit a discrete GPU in there. Especially with smaller internal volume than the 15 inch and the need for a big battery.
I do think Apple can fit it all in there. The biggest problem will be heat. Don't current 13" MBPs only have 1 internal fan?
Not if they try using IGZO tech which will save them throwing a massive battery in there = saved space.
And 13" MBPs have 2 fans I believe...