@gravitas Right, that makes more sense now :)
If you compare the browser samples of every font on Typekit, you might change your mind. OSX font rendering is well known to be heavier than Windows. I’m fairly confident that you aren’t experiencing an issue at all, simply differences in rendering. I work with type regularly, and this all sounds completely normal to me.
On Mac OS and iOS, we hardly have any control over the rendering, which is acceptable (since it’s generally very reliable). One problem is that fonts generally render too heavy. Maybe some day, Web font services can improve the consistency by serving slightly heavier or lighter fonts depending on the platform.
As further evidence, here is the default rendering of Merriweather on Firefox (left is OSX, right is Windows): http://cl.ly/1T0v262c3v3T29220F1z