The ability of AppleScan to enlarge and reduce images is based on the
resolution of the scan. Page 196 of the "Apple Scanner Owner's Guide"
outlines this relationship and states:
"The Reduce/Enlarge process works in tandem with the resolution you are
using. Reduce/Enlarge changes the effective scan resolution, and the
values you can choose vary depending on the resolution setting. If you
reduce an image, the scanner detects fewer dots per inch; if you enlarge an
image, the scanner detects more dots per inch."
If the scanned image resolution is set to 300 dpi, it is not possible to
increase the size of the image. This requires the Apple Scanner to have a
resolution beyond 300 dpi. However, if the scan resolution is set to 150
dpi, it is then possible to enlarge the scanned imaged to 200%, or, in
effect, 300 dpi for the Apple Scanner.
The complete description of how Reduce/Enlarge works in conjunction with
resolution is on pages 194 to 198 of the "Apple Scanner Owner's Guide".