I don't think you're using units that could allow anyone to detect whether it's caffeine quantity or bioavailability that's at play. It depends on how you dose your espresso and drip coffee. I don't drink espresso but I think people are dosing something like 20 grams per shot, whereas for pourover they're dosing 18 grams of water per gram of coffee. For a standard "coffee cup" (which is 5 ounces of water that results in 4 ounces of coffee = 148 g of brew water), you're using 8g of coffee. Where I'm going with this is if your portafilter takes 40g of coffee for a double shot espresso, that is more coffee than 32g of coffee you'd use to brew 4 "cups", so you're just drinking more coffee. But, standard cups are a dumb unit (my coffee mug holds 400mL of water), so it's possible you aren't using that, and the espresso has a better caffeinating effect for the same amount of caffeine. We can't know because the words and units are conflated in so many ways (there's a cup, which is a drinking vessel, there's the cooking cup which is like 230mL, and there's the coffee cup which is 148mL of water in to get 118mL of coffee out).
I'm not a stickler for metric vs. imperial (I use both), but "cup" is a terrible terrible unit that needs to die. Use grams for anything coffee-related.