text: It turns out that as the beans roast, they lose water and also puff up slightly—and the longer the roast time the more pronounced these effects. Dark roast beans will thus weigh less (and be slightly larger) than light roast beans. When the ground beans are measured by volume, the light roast particles will be denser, weigh more, and contain more caffeine than the dark grinds, producing a more caffeinated brew.
  THE BOTTOM LINE: The only way to ensure that you’re getting the same amount of caffeine with different roasts (all other variables being equal) is to weigh coffee. If you measure by volume, you’ll end up with more buzz with a light roast than with a dark roast.