Credit-card growth has slowed in recent years as debit cards and mobile payments have risen in popularity, which may be behind the rise in large sign-up bonuses for credit cards, said Rob Markey, the global leader of Bain & Company's Customer Strategy & Marketing practice. This also may be due to banks figuring out that large sign-up bonuses are effective, and directing funds to those instead of other forms of marketing. "If your product is good enough to generate buzz and word-of-mouth, why not just turbocharge it?" Markey said.