发横财: make a fortune in a devious way; strike it rich; get [have] a windfall; get rich by foul means; strike oil; ill-gotton gains
大发横财: make staggering profits; amass ill-gotten wealth [riches]; get rich illegally; make a pot of money by foul methods; make piles of money; reap [acquire] exorbitant [huge] profits [gains] by