If webfonts look fatter or thinner depending on your OS ...

You've most likely discovered Webkit's new browser-specific anti-aliasing strategy, which differs from browser to browser and from OS to OS, making a sticky soup of fine webfonts on Macs for example. To circumvent this, use the following webkit-rule.

This rule can have three values: none (which deactivates antialiasing and looks mostly terrible), antialiased (which is the default on Windows for example and only applies a little antialiasing) and subpixel-antialiased (which applies heavy antialiasing and simply looks terrible with fine fonts).