A handy rule: log of 2 to base 10 is a bit more than 0.3, so divide the power of 2 by 3 (i.e. approximately multiply by 0.3), and you'll get an approximation of the power of 10. By this, 2^128 is in the region of 10^40, a lot less than 10^79.

(It's actually about 10^38.5.)