HTML: Alter margins of submit button?

Why do submit buttons have a 20 pixel or so margin on each side of the text? I mean is it really necessary to have all that open space– I am not looking for a place to graze cattle on.

I am putting in an add to shopping cart button, and I could fit it into the space I need to, if I just got rid of those pesky margins. Is there a way?

I know I could use an image instead, but let’s face it, making a cool button with css and text is a way better way for me to waste time.