>> If Banias and the TM5800 are both manufactured using .13 micron process and Banias has roughly 77 million transistors while the TM5800 has 13 million, why is the die size of Banias only twice as big (100 mm2) as the TM5800 (55 mm2)? <<
I believe the answer is that the die sizes you quoted include the real estate devoted to the cache.
If you subtract the portion for the cache from the size of each processor, you are left with the size of each processor core. You will find that the core size for Banias is much greater than 2X the core size for the TM5800.
(I’ll defer to any of the resident hardware experts if I misspoke about this.)