Social icon images – small and too light

Hello
The social icon images on my icon buttons are very small compared to the size of the button. I’ve tried the different sizes of icons, but the image inside the button is always the same. Is it possible to make the f of Facebook (for example) bigger inside the box it sits in?
If not, I am using the light version of the icons, and the box that the icon image lives in almost completely blends into the background. Is it possible to still use the light setting, but force the background to be slightly darker?
See footer on http://juniorchoir.lovemusic.org.uk
With thanks
R