An “em” is a typographical measurement which goes back to letterpress days. It refers to the height of the piece of metal (or wood) that any letter in a given type font is mounted on (here I am using type font in its true meaning which is one size of one style of type). Typically the capital M was a square piece and therefore the measure became known as an “em”. Another typographical measurement is the “en” which is one-half of an em.

X-height does refer to the height of a small letter but more correctly refers to the height of the type font characters which do not have ascenders or descenders (i.e., b,d,g,p, etc.). the height is measured from the baseline to to top of the highest letter. That means that in some fonts (Garamond comes to mind) the x-height is higher than the small “x”, because of the height of the rounded letters (a, c, m, etc.).

Few designers today have handled lead type or even studied the history and origins of the typefaces they use. Many would argue it makes little difference. I think we would have fewer illegible pages.

Some devices are taking it into their own hands, like the iPhone artificially bumping up sizes for readability.