What you’re probably thinking of is the US airline specific “comfort/extra/plus” add-on that is priced 10%-20% extra. There is a few more inches legroom but the seats are the same small economy seats.
The former uses larger seats and is typically available on long-hauls. The seat is somewhat similar to old-style business class seats, or modern domestic first class seats. It is not priced at 10% extra, it is usually close to double.
The latter is priced at around 10% extra, but is the same old economy seat just with a couple of inches more legroom.
It would be nice to have to pay only 10% extra to get 10% more space, but the lack of scaling is by design on the airlines part, and textbook market segmentation.
Edit: this article has pictures showing the difference: https://www.cntraveler.com/story/whats-the-difference-betwee...