5B transistors is good for a full Tahiti (32 CUs, 384-bit memory controller, dedicated video hardware, etc.) plus 8 jaguar cores (assuming some 40M transistors each?) and still have some 500M transistors left.
Also a prohibitively large and hot die.
A fair amount of transistors are sunk into the SRAM arrays, 2 x 4MB CPU caches and the 32MB ESRAM is going to be 2B transistors alone, add 2B for a 7790-ish GPU, and you have 1B left over for the CPUs, the audio DSP block, the Kinect DSP, clock network and repeaters.
Sounds about right.
Cheers