Different brew methods extract differing amounts from the coffee, so the amount of water is indeed consequential. Same grams of coffee in to a pour-over vs espresso would result in different amounts of extracted coffee material consumed.
By comparison, the same 18g brewed using a pour over method should yield an 8oz cup of coffee with … likely around 170mg of caffeine.
Check out this James Hoffmann video specifically on this topic: https://youtube.com/watch?v=etnMr8oUSDo