In order to achieve better spacing and line breaking, pdfTeX can expand fonts. The user decides how much stretching and shrinking is allowed for each desired font, and in what increments. However, another way would be to set each line knowing that the entire line can be stretched or shrunk via a pdf transform (horizontal scaling). This has two advantages: (1) the adjustment is continuous, so discreteness effects of increments are avoided; and (2) all fonts, including all math, are handled simultaneously. This also seems easier to implement. A disadvantage is that certain characters ought to be expanded in different amounts; pdfTeX used
\efcode for this purpose. Note, though, that pdfTeX does not use different glyphs: according to the pdfTeX user manual (p. 22), "For practical reasons pdfTeX does not use such huge glyph collections; it uses horizontal scaling instead".
There are trade-offs. Can anyone explain why the method I suggest is not used or is not better? Is there code somewhere that uses my method?