There are legal problems with messing with fonts like Arial, so you would be safer using my font downloadable from the tutorial section, or some other free font. Altering Existing Fonts
Probably the problem you are seeing is from “hinting.” You will find lots of references on all of these forums. Take some time to read them. John Cork and Bhikkhu Pesala have great articles.
When you modify a hinted font, part of the hinting goes away. Hinting helps on the computer screen, but makes no difference to printed output. You can read about “Volt” and try to do your own hinting, or pay someone to do it.
So far, most of us don’t want the bother, but Erwin is thinking about adding it.