I remember reading somewhere that the amount of caffeine in the cup is also proportional to the brewing time, therefore an espresso may not actually contain that much caffeine in the end (for the same initial amount of ground coffee, of course)

When I was in Organic Chemistry in College we demonstrated this experimentally. A cup of drip coffee had more caffeine than a shot of espresso. We were college kids so I'm not sure how consistent our how methodology was but the result was pretty consistent across all of us.