I believe it is due to how the machine works :
* on an espresso machine : the hot water is pressurized and passes quickly through ground coffee. It doesn’t have time to extract that much caffeine.
* in a filter drip machine : hot water drips from the top inside the funnel filled with ground coffee. It has to slowly go through just by gravity. Caffeine has more time to dissolve into water.
As others have mentioned the main reason for drip coffee having more caffeine is the longer contact time with water.
However I need to mention that you are very wrong about the amount of coffee beans needed to brew a drip coffee. A spoonful per cup is enough only if you are brewing an instant. Instant coffee is not a finely ground coffee, it’s actually dehydrated coffee that has already been extracted.
I make a double shot with around 16g of coffee and 250ml of drip coffee with around 15g of coffee, so they are actually very comparable in terms of total coffee.
The difference in brew time and pressure means that you have different ratios of soluble compounds, but I actually think that the amount of caffeine is around the same for the same amount of coffee, e.g. around 100mg. Technically, the longer brew time would mean that more of the relatively non-polar caffeine (compared to things like sugars) would be extracted in drip coffee, but most of the caffeine is extracted fairly early on, so the actual difference is fairly minimal for the same starting mass of coffee. Some people do prefer stronger drip coffee of weaker espresso, meaning there can be a lot of overlap.
Maybe I missed it in the responses, but a shot of espresso is also far SMALLER than a cup of drip.
[By the numbers from the Mayo Clinic](https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/caffeine/art-20049372), a cup of drip with 96mg of caffeine is 8 ounces, while the espresso (64mg in their chart) is 1 ounce.
To directly compare,
1 ounce espresso: 64 mg
1 ounce drip: 12 mg
8 ounces drip: 96 mg
8 ounces espresso: 512 mg
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