Как уже упоминалось в W3C CSS2 шрифтах документ:
<relative-size>
A <relative-size> keyword is interpreted relative to the table of font sizes
and the font size of the parent element. Possible values are: [ larger | smaller ].
For example, if the parent element has a font size of 'medium', a value of 'larger'
will make the font size of the current element be 'large'. If the parent element's
size is not close to a table entry, the UA is free to interpolate between table
entries or round off to the closest one. The UA may have to extrapolate table values
if the numerical value goes beyond the keywords.
И ниже этого говорит:
Note 2. In CSS1, the suggested scaling factor between adjacent indexes was 1.5, which
user experience proved to be too large. In CSS2, the suggested scaling factor for a
computer screen between adjacent indexes was 1.2, which still created issues for the
small sizes. Implementation experience has demonstrated that a fixed ratio between
adjacent absolute-size keywords is problematic, and this specification does not
recommend such a fixed ratio.
Надеюсь, это ответит на ваш вопрос.