Twenty twelve remove blank space under caption

I’m running theme twenty twelve, I would like to get rid of the huge space that appear if I add the caption to a picture.
Default the caption text is 12px, and the “box” where it is is 24px (2 lines of 12px).
So I want to get rid of the second line and change the font size to 8px.
At the moment I can simply change the font size, with the editor tool, but I’m not able to remove the 24px space.
I have a child theme and did lot of try modifying the style.css but nothing was working.

thank you