differences between (IIS PHP GD) & (Apache PHP GD)

Hi, i just made a little site with a nice counter, i use gd to draw the number in a chosen TTF no problem for that, but when i run the same source on IIS the string is much bigger and doesn't fit in the image anymore. i can adapt it again but it will be much smaller in apache then. Somebody knows something about this strange thing. i didn't searched much but i think under iis gd take size as points and under apache as pixels.. Regards