As we have seen, all dimensions in a TFM file are expressed in design
size units. The idea is to make it simple for TeX to scale a font.
Indeed, when a TeX user does \font\foo=cmr10 at 12pt or
\font\foo=cmr10 scaled 1200, all TeX has to do is change the
font’s design size. The drawback, on the other hand, is that in order to
actually use the various font dimensions, they need to be multiplied by
the design size in question every time.