I think you need the readjust the padding-left and padding-right on the button. I would advise to do this the right way with new class on the button and adjustments in a _button.scss file.
If you must you could add ‘padding-left: 0’ in the style box, but usually you'll get shot by UI guys for doing so.
I’m kind of new to Mendix but have you tried to update the settings in the “Design properties”?