I think shikkediel is right that the font appearing bolder is just based on our perception.
The letter moving issue is most apparent in firefox (didn’t see it at all in chrome, ie was pretty ok)
I think it is just a result of how FF renders and scales fonts.
Also, the scaling and transitions are not running in IE or FF (as you have it coded). I noticed that you had a missing semi colon, and I think there is something goofy going on with your prefixes…