The -s flag controls the scale of the result. The default scale is 1, which results in one pbm pixel producing a 3x3 square of PostScript pixels. On a 300 dpi printer such as the Apple Laserwriter, this makes the output look about the same size as the input would if it was displayed on a typical 72 dpi screen. To get one pbm pixel per PostScript pixel, use "-s 0.333333".
Note that there is no pstopbm tool - this transformation is one-way, because a pstopbm tool would be a full-fledged PostScript interpreter, which is beyond the scope of this package.
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