In Windows, proof usually means good, and draft means poor but quick. The screen driver will never double or triple the size of a font for proof quality. However, it will double or triple the size of smaller fonts to get the exact size it wants for draft quality. If the correct size is available, the screen driver will always choose it.
Doubling and tripling occur only when the exact size is not available. For example, assume the only screen sizes are 10 and 19 lines, and the application wants a 20-line screen font. In proof quality, the screen driver would choose the 19-line font. In draft quality, the screen driver would double the 10-line font.