There are other factors too, but the longer coffee beans are roasted the less caffeine it will have. French Roast (the longest/darkest roasted coffee) has less caffeine but appears stronger tasting only because of the roasting time. Beans used for espresso are generally roasted longer than beans that would be used for drip coffee, Starbucks uses the same varietal (or really a blend) of coffee for their espresso, Italian, and french roasts, only the roasting time of the beans are changed.