Agreed, we have no idea if the real "J-31" will have the same dimensions as the current 31001 demonstrator.
In particular, I think I wouldnt' be opposed to J-31 having a larger, even bulbous weapons bay like F-35, even if it isn't as stealthy or sleek as its current weapons bay. After all, it is meant to be a multirole fighter and it should have the internal volume to carry a pair of powered, decent sized stand off munitions.