Another ‘how do you know that’? From what I can see they have the same trunk space. The Prius has AWD, the Prime does not, with that room taken up by the (single) larger battery.
edit: the Toyota web site lists identical cargo space for both, 20.3 ft3.
I’m in the market for something like that, but I’d go with the Corolla Cross hybrid, great room, visibility, utility, and mpgs.