I've read this, but didn't actually provide a solution, and the issue was reversed from my problem.
So I'm using Roboto Slab with 400 weight, 1em size, which renders nicely on Windows but incredibly thick on OS X.
I only realised this when I bought a MacBook Air.
Is there a way I can reduce the weight if a user is viewing on OS X? Or something like em (vs px) for weight?