Except that isn't how font metrics work—the size of the type on the body is not necessarily equivalent between two different fonts. There is no stanard that says that 12pt Times shares any vertical metrics with 12pt Helvetica, or 12pt anything else.

The most informed comparision can be made when the x-height is optically equivalent. The other option, cap height, isn't as useful because it is then harder to judge the lowercase (a majority of the letterforms).

I think what jack's saying is that "font-size" to some extent is arbitrary. It's not scientific or worthwhile to compare fonts across a specific size if they have different x-heights. The only meaningful comparison is to evaluate legibility for fonts that are "relatively" the same size. For example, if Interface 12pt is equivalent in x-height to Roboto at 12.5pt, and Roboto is more legible, and legibility is the goal, then obviously the user should pick Roboto at 12.5pt (all things considered).