Set two different typefaces at the same point size — say, 12pt — side by side, and one will often look noticeably larger than the other. That's not a bug or an inconsistency. Point size measures a historical, largely invisible property of a typeface's metal-type ancestry, while the thing your eye actually uses to judge a letter's readable size is a completely different measurement: x-height.
What point size actually measures
Point size traces back to physical metal type, where each letter sat on a small rectangular block of a fixed height, tall enough to accommodate the tallest ascender (like the vertical stroke of a "b" or "l") and the lowest descender (like the tail of a "g" or "y") with a small margin of clearance on top and bottom. Point size measures that block, not the visible ink on the page — and different typefaces were designed with different amounts of built-in clearance and different proportions between their letters and that surrounding block, a relationship that survives unchanged in digital fonts even though the physical metal block no longer exists.
X-height: the part your eye is actually using
X-height is the height of a typeface's lowercase letters that have no ascenders or descenders — named for the lowercase "x," which is a clean, flat-topped, flat-bottomed reference letter — and it's the measurement that most closely tracks how large and readable a typeface actually looks at a glance, because lowercase letters without ascenders or descenders make up the bulk of ordinary text. A typeface with a large x-height relative to its point size looks bigger and is often more legible at small sizes, particularly for body text or for signage meant to be read quickly, while a typeface with a small x-height relative to its point size can look markedly smaller and finer even set at the identical point size. This is why a type designer choosing a font for a small caption or a highway sign cares far more about x-height than about the point size number itself.
What we're still unsure about
The mechanics of point size and x-height are well documented typographic history, not a live debate. What's genuinely less settled is exactly how much of a typeface's real-world legibility comes from x-height specifically, versus other interacting factors — letter spacing, stroke contrast, counter (the enclosed or partially enclosed space inside letters like "o" or "e") size, and the reading context itself, including screen resolution and viewing distance. Legibility research generally treats x-height as one significant factor among several rather than the single dominant one, and designers still rely heavily on testing and experience alongside any measurable typographic rule.
This sits inside Typography & Type Systems, one of seven topics in Design, one of five domains in Art, one of seventeen subjects the app can quiz you on.