Anyway,
The time defraging takes hugely depends on how badly defraged the harddrive it is.
It isn't a linear relationship. It doesn't depend on how big the harddrive is, though that does affect some stuff.
Considering that your computer had an extraordinary performance boost after defraging that 80 gig harddrive, then that harddrive must've been pretty badly defraged.
So, no, your math was not wrong, but you also forgot to examine the fact that harddrives have different levels of defragmentation.