I am just going to play with Numbers a bit.
Apple needed a Bigger iPad for Certain type of Activities. Cashier , Display is one of them, designer, medical uses etc.
12.9 " Res at doubling of iPad Air, 4096 x 3072 = 396 PPI
Another Rumored iPhone Devices at less then 5"
4.95" Res at 1.5x of the iPhone 5 , 1704 x 960 = 396 PPI
I am going to guess and play by the rumors, the bigger iPad Pro wont be a Huge hit ( comparatively to iPad Air and Mini ). But...