Are you kidding me...? are you actually arguing that the Fatman should be equal to a small lunch box sized pack of 200year old C4?
To be fair, a block of C4 the size of the ones in Fallout: New Vegas would be obcenely powerful. You could knock out a bridge with something that size. Also, the Fatman realistically would be impossible to use without killing yourself in the process.
http://en.wikipedia.org/wiki/Davy_Crockett_(nuclear_device) would be the closest real life example of what the Fat Man would be like but the Fat Man is also smaller and more portable. The nuclear reaction would be much smaller, and therefore less powerful.
It couldn't hurt to give it a little boost though