This analogy doesn't make sense to me.
Generosity doesn't seem to have a connection to frugality- you can be extremely generous and frugal or you can not be generous at all and frugal. So it is impossible to decide the word that has the same connection to Stingy, because it could be on either side of the spectrum.
To answer the question in the thread title, I would say spendthrift is a good word, but would also like to offer up "prodigal".