This may be true if you NEVER ask for a font with a particular size, but If you do
then you have to make your own adjustment. My internal logic for font size is
calibrated for a nominal 12 point font on a 96 DPI screen, and when I ask for
a derived font, I ask for <actual DPI>*size/96
If you're trying to do something genericly "bigger/smaller", beware that it can be quite difficult
to determine what size a font you're given is, as some starting fonts (even truetype fonts)
do not have a reliable size, and that size they claim to be doesn't correspond directly to the size
you'd get if you asked for one the size they say they are.