Well, I don't know your theme, but it's one of those line-height attributes in your style.css. Open up your style.css and click Control-F and search for line-height. Adjust one of them until you find the right one. It might be under mainwrapper or something, but I'd have to know your particular theme. For sure, though, it's some line-height attribute in that file.
For future reference, it's even better if your theme has a custom CSS option. Then just paste the CSS in there, which will overwrite your style.css. This way, you're not altering your theme's core WordPress files, and so when the next upgrade comes out, you won't lose all your changes.
And don't forget to clear your browser's cache.