Light/medium isn't really a higher caffeine content. It is true that caffeine cooks away (to an extent) in the roasting process and because of that on a per bean basis, light roasts will have more caffeine.

That being said, we typically measure coffee by weight and the more you roast coffee the lighter it becomes (moisture loss). This means that you have more physical beans for the same given weight which offsets the loss of caffeine. I.e. 20 beans and light roast coffee has more caffeine then 20 beans of dark roast, but 20g of light roast coffee has about the same caffeine as 20g of dark roast