With a lighter roasted coffee, more of the caffeine that was in the coffee originally is still in the coffee. Speciality coffees measure the amount of coffee powder used for brewing in weight units flavor, and a dark roast’s density can be substantially lower than a light roast’s.
That means a dark roast will yield more beans per gram of coffee than a light roast. If you are measuring out 20 grams off coffee for both lighter roast and a darker roast. The darker roast will have more beans at 20 grams due to less density. While the darker roast may have less caffeine per bean, you have to have more beans to equal the same amount of coffee.
So the difference in caffeine is therefore minimal when comparing light roast to dark roast. Most important factor in the amount of caffeine in coffee is the type of coffee being roasted and the altitude at which they were grown.
