Why did 15g sugar pancakes taste sweeter than 100g sugar cupcakes?

Can't say for sure, but if I had to venture a guess, it's likely the surface area and browning. Carmelizing the sugars makes them sweeter and have a more distinctive and noticeable taste. In addition, flour is a starch which is a complex carbohydrate. When you break down complex carbs through cooking, they become simpler carbs like sugars. I'm not certain, but I'm pretty sure that heats high enough to cause the browning is also enough to cause some of that break down of the carbs at the surface of the pancake.

P.S. If you have the self control, you can demonstrate for yourself this carb breakdown into sugars. Take a saltine cracker, chew on it for something like 5 minutes without swallowing (hard to do, honestly). It will turn into a mush and slowly start being broken down by the enzymes in your saliva. After long enough, the complex carbohydrate molecules will be broken down into glucose molecules and the mush will start to taste noticeably sweet. Kinda trippy.

must be the 200g of syrup you bathe them in

The other ingredients matter. Certain other flavors can mask or enhance sweetness.

https://www.spiceography.com/too-much-sugar/

Is that for the same weight, volume, what?

Did you put maple syrup on the pancakes? That would probably explain it 😉

I noticed this with coffee a while back. Was putting several tablespoons teaspoons of sugar in to make it palatable but got distracted one day and only put in two, it was sweet and it was much better tasting

How much smaller were the sweeter pancakes than the other ones?

I had the same thought when I ate chupa cups. The small one only has 6 grams of sugar, the big one has 12 grams, but they are equally sweet, how can that be...