[FoundersGrotesk]

I think you’re referring to “optical sizing” in fonts. In essence, fonts at small sizes are more robust and large sizes have more detail. Very much like comic books illustration, for example. If you blow up a small face on a comic strip and compare to a big face, the big face will have a lot more fine detail. The small face will be simplified and more gestural, so it can be “read” at small sizes.

Tobias Frere-Jones has some blog posts on Typeface Mechanics if you’re interested:

https://frerejones.com/blog/typeface-mechanics-001 https://frerejones.com/blog/typeface-mechanics-002