Black and white bmp results in grey pdf


I'm experiencing a strange problem converting (scanned) black/white pictures (bmp) to pdf on Windows 7 pro.

I used to do this on a machine with Windows XP Pro (up-to date) with no problems. Using default settings it turns a 3,6 MB bmp (1 bit black-and-white) into a reasonable 220 kB pdf.

Recently I bought a new pc with Windows 7 pro (retail), installed pdf-creator and started using it with the default settings. Accidentally I converted the same 1 bit b/w bmp, and this appeared to be 1,36 MB!! It appeared that the picture within the pdf was no longer black and white, but also had various grey colors on the transitions from black to white.

I've made a clean install on both pc's (Win xp and 7) used pdf-creator 1.4.2, checked and double-checked the settings within pdf-creator, but i can't find the problem. The past few days í have checked it on 4 other different machines. To me it seems to be a combination of windows 7 and pdf-creator. Each Win-Xp machine returns a small (220k) pdf and the Win-7 machines a larger (1,36 MB) file. Altough the printing-wizard UI is slightly different, the various settings are the same.

Also I have been playing with the various settings but besides the reducing filesize, the grey remains where the original is pure black and white

What am I missing? Any suggestions?

Kind regards,