Though things are changing in the type world, I think optical sizing is still undervalued by type users and type providers. Optical sizing is varying the shapes of glyphs to compensate for the differences in perception that happen when fonts are set at different sizes. In the words of Tim Ahrens (who literally wrote the book on the subject), “Characters intended for use in small sizes typically show an increased width and x-height, reduced stroke contrast and looser spacing.”
When thinking about these kind of adjustments we usually have text types in mind—those intended for continuous reading. Less attention has been paid to using optical sizes in display types, but Feneon is a great example of a display type that benefits from optical sizing, though in this case it does not relate to x-height or spacing.
The letters on the left show Feneon, Feneon Single, and Feneon Dense with no optical sizing. There, the letters that are big look relatively too bold, and the small ones way too light. On the right, we have the same glyphs but with each one set to the optical size that matches its point size. That set demonstrates the impression of consistency that optical sizing confers. After all, a neon bender uses the same stock of glass tubing whether the letter they’re going to make is big or small!