However, you certainly can intentionally scale the image before printing to the fax driver. Scanning a small page at 400 dpi and scaling to 200 dpi before printing will double its size, as per normal scaling rules. The difference in Normal and High fax resolution is the way the fax machine steps its paper motor. At High resolution, motor steps of 196 lines per inch are used for each scan line, and every line is sent. For Normal resolution, double sized vertical steps of 98 steps per inch are used, meaning Normal mode simply sends every other scan line, and omits remaining lines.
Normal resolution requires larger text to be readable.
High resolution therefore takes twice the Normal telephone time to transmit, because all scan lines are sent, twice as many. But solid color areas of all white or all black pixels compress extremely well, solid color areas are a very small cost in fax speed. For Normal fax mode, 203x98 dpi, we should still scan at 200 dpi, because it is still 200 dpi horizontally. If we specify High resolution, the fax modem will send the best mode the other guy can handle, but that may not be High resolution. Not all fax machines are plain paper machines either.