Look at the back of the card u ll see that actual card is way smaller than it is with that added cooler. I believe that is fan segment of cooler that protrudes from the back which adds to the length
I think it's mostly due to not wanting another launch were reference cards are far too hot. Hawaii, and the Nano were far too hot on reference coolers, and latter was already downlocked; but could still throttle.
With this design they have a decent junk of alu with a copper slug on the core, and also cooling for the rest of the PC.
I'm sure there will be smaller AIB cards to take advantage of the shorter pcb in the end, but a safer best for reference seems wise after the years of negative stereotypes about how hot AMD reference cards are.