Indeed, I never thought to look at the packaging beyond a casual glance at the labelled filter size. As it turns out, my filters were the correct size all along, and I just wasn't making coffee ...

According to the article's caffeine chart, a one-ounce shot of espresso has 30-50 mg of caffeine versus the 140-200 mg in drip brewed Robusta coffee or 75-130 mg in Arabica coffee (per 6 ounce cup).